On Sun, Nov 11, 2012 at 03:40:24PM -0600, Alan Cox wrote: > On Sat, Nov 10, 2012 at 7:20 AM, Konstantin Belousov > <[email protected]>wrote: > > > On Fri, Nov 09, 2012 at 07:10:04PM +0000, Sears, Steven wrote: > > > I have a memory subsystem design question that I'm hoping someone can > > answer. > > > > > > I've been looking at a machine that is completely out of memory, as in > > > > > > v_free_count = 0, > > > v_cache_count = 0, > > > > > > I wondered how a machine could completely run out of memory like this, > > especially after finding a lack of interrupt storms or other pathologies > > that would tend to overcommit memory. So I started investigating. > > > > > > Most allocators come down to vm_page_alloc(), which has this guard: > > > > > > if ((curproc == pageproc) && (page_req != VM_ALLOC_INTERRUPT)) { > > > page_req = VM_ALLOC_SYSTEM; > > > }; > > > > > > if (cnt.v_free_count + cnt.v_cache_count > cnt.v_free_reserved || > > > (page_req == VM_ALLOC_SYSTEM && > > > cnt.v_free_count + cnt.v_cache_count > > > cnt.v_interrupt_free_min) || > > > (page_req == VM_ALLOC_INTERRUPT && > > > cnt.v_free_count + cnt.v_cache_count > 0)) { > > > > > > The key observation is if VM_ALLOC_INTERRUPT is set, it will allocate > > every last page. > > > > > > >From the name one might expect VM_ALLOC_INTERRUPT to be somewhat rare, > > perhaps only used from interrupt threads. Not so, see kmem_malloc() or > > uma_small_alloc() which both contain this mapping: > > > > > > if ((flags & (M_NOWAIT|M_USE_RESERVE)) == M_NOWAIT) > > > pflags = VM_ALLOC_INTERRUPT | VM_ALLOC_WIRED; > > > else > > > pflags = VM_ALLOC_SYSTEM | VM_ALLOC_WIRED; > > > > > > Note that M_USE_RESERVE has been deprecated and is used in just a > > handful of places. Also note that lots of code paths come through these > > routines. > > > > > > What this means is essentially _any_ allocation using M_NOWAIT will > > bypass whatever reserves have been held back and will take every last page > > available. > > > > > > There is no documentation stating M_NOWAIT has this side effect of > > essentially being privileged, so any innocuous piece of code that can't > > block will use it. And of course M_NOWAIT is literally used all over. > > > > > > It looks to me like the design goal of the BSD allocators is on > > recovery; it will give all pages away knowing it can recover. > > > > > > Am I missing anything? I would have expected some small number of pages > > to be held in reserve just in case. And I didn't expect M_NOWAIT to be a > > sort of back door for grabbing memory. > > > > > > > Your analysis is right, there is nothing to add or correct. > > This is the reason to strongly prefer M_WAITOK. > > > > Agreed. Once upon time, before SMPng, M_NOWAIT was rarely used. It was > well understand that it should only be used by interrupt handlers. > > The trouble is that M_NOWAIT conflates two orthogonal things. The obvious > being that the allocation shouldn't sleep. The other being how far we're > willing to deplete the cache/free page queues. > > When fine-grained locking got sprinkled throughout the kernel, we all to > often found ourselves wanting to do allocations without the possibility of > blocking. So, M_NOWAIT became commonplace, where it wasn't before. > > This had the unintended consequence of introducing a lot of memory > allocations in the top-half of the kernel, i.e., non-interrupt handling > code, that were digging deep into the cache/free page queues. > > Also, ironically, in today's kernel an "M_NOWAIT | M_USE_RESERVE" > allocation is less likely to succeed than an "M_NOWAIT" allocation. > However, prior to FreeBSD 7.x, M_NOWAIT couldn't allocate a cached page; it > could only allocate a free page. M_USE_RESERVE said that it ok to allocate > a cached page even though M_NOWAIT was specified. Consequently, the system > wouldn't dig as far into the free page queue if M_USE_RESERVE was > specified, because it was allowed to reclaim a cached page. > > In conclusion, I think it's time that we change M_NOWAIT so that it doesn't > dig any deeper into the cache/free page queues than M_WAITOK does and > reintroduce a M_USE_RESERVE-like flag that says dig deep into the > cache/free page queues. The trouble is that we then need to identify all > of those places that are implicitly depending on the current behavior of > M_NOWAIT also digging deep into the cache/free page queues so that we can > add an explicit M_USE_RESERVE. > > Alan > > P.S. I suspect that we should also increase the size of the "page reserve" > that is kept for VM_ALLOC_INTERRUPT allocations in vm_page_alloc*(). How > many legitimate users of a new M_USE_RESERVE-like flag in today's kernel > could actually be satisfied by two pages?
I am almost sure that most of people who put the M_NOWAIT flag, do not
know the 'allow the deeper drain of free queue' effect. As such, I believe
we should flip the meaning of M_NOWAIT/M_USE_RESERVE. My only expectations
of the problematic places would be in the swapout path.
I found a single explicit use of M_USE_RESERVE in the kernel,
so the flip is relatively simple.
Below is the patch which I only compile-tested on amd64, and which booted
fine.
Peter, could you, please, give it a run, to see obvious deadlocks, if any ?
diff --git a/sys/amd64/amd64/uma_machdep.c b/sys/amd64/amd64/uma_machdep.c
index dc9c307..ab1e869 100644
--- a/sys/amd64/amd64/uma_machdep.c
+++ b/sys/amd64/amd64/uma_machdep.c
@@ -29,6 +29,7 @@ __FBSDID("$FreeBSD$");
#include <sys/param.h>
#include <sys/lock.h>
+#include <sys/malloc.h>
#include <sys/mutex.h>
#include <sys/systm.h>
#include <vm/vm.h>
@@ -48,12 +49,7 @@ uma_small_alloc(uma_zone_t zone, int bytes, u_int8_t *flags,
int wait)
int pflags;
*flags = UMA_SLAB_PRIV;
- if ((wait & (M_NOWAIT|M_USE_RESERVE)) == M_NOWAIT)
- pflags = VM_ALLOC_INTERRUPT | VM_ALLOC_NOOBJ | VM_ALLOC_WIRED;
- else
- pflags = VM_ALLOC_SYSTEM | VM_ALLOC_NOOBJ | VM_ALLOC_WIRED;
- if (wait & M_ZERO)
- pflags |= VM_ALLOC_ZERO;
+ pflags = m2vm_flags(wait, VM_ALLOC_NOOBJ | VM_ALLOC_WIRED);
for (;;) {
m = vm_page_alloc(NULL, 0, pflags);
if (m == NULL) {
diff --git a/sys/arm/arm/vm_machdep.c b/sys/arm/arm/vm_machdep.c
index f60cdb1..75366e3 100644
--- a/sys/arm/arm/vm_machdep.c
+++ b/sys/arm/arm/vm_machdep.c
@@ -651,12 +651,7 @@ uma_small_alloc(uma_zone_t zone, int bytes, u_int8_t
*flags, int wait)
ret = ((void *)kmem_malloc(kmem_map, bytes, M_NOWAIT));
return (ret);
}
- if ((wait & (M_NOWAIT|M_USE_RESERVE)) == M_NOWAIT)
- pflags = VM_ALLOC_INTERRUPT | VM_ALLOC_WIRED;
- else
- pflags = VM_ALLOC_SYSTEM | VM_ALLOC_WIRED;
- if (wait & M_ZERO)
- pflags |= VM_ALLOC_ZERO;
+ pflags = m2vm_flags(wait, VM_ALLOC_WIRED);
for (;;) {
m = vm_page_alloc(NULL, 0, pflags | VM_ALLOC_NOOBJ);
if (m == NULL) {
diff --git a/sys/fs/devfs/devfs_devs.c b/sys/fs/devfs/devfs_devs.c
index 71caa29..2ce1ca6 100644
--- a/sys/fs/devfs/devfs_devs.c
+++ b/sys/fs/devfs/devfs_devs.c
@@ -121,7 +121,7 @@ devfs_alloc(int flags)
struct cdev *cdev;
struct timespec ts;
- cdp = malloc(sizeof *cdp, M_CDEVP, M_USE_RESERVE | M_ZERO |
+ cdp = malloc(sizeof *cdp, M_CDEVP, M_ZERO |
((flags & MAKEDEV_NOWAIT) ? M_NOWAIT : M_WAITOK));
if (cdp == NULL)
return (NULL);
diff --git a/sys/ia64/ia64/uma_machdep.c b/sys/ia64/ia64/uma_machdep.c
index 37353ff..9f77762 100644
--- a/sys/ia64/ia64/uma_machdep.c
+++ b/sys/ia64/ia64/uma_machdep.c
@@ -46,12 +46,7 @@ uma_small_alloc(uma_zone_t zone, int bytes, u_int8_t *flags,
int wait)
int pflags;
*flags = UMA_SLAB_PRIV;
- if ((wait & (M_NOWAIT|M_USE_RESERVE)) == M_NOWAIT)
- pflags = VM_ALLOC_INTERRUPT | VM_ALLOC_WIRED;
- else
- pflags = VM_ALLOC_SYSTEM | VM_ALLOC_WIRED;
- if (wait & M_ZERO)
- pflags |= VM_ALLOC_ZERO;
+ pflags = m2vm_flags(wait, VM_ALLOC_WIRED);
for (;;) {
m = vm_page_alloc(NULL, 0, pflags | VM_ALLOC_NOOBJ);
diff --git a/sys/mips/mips/uma_machdep.c b/sys/mips/mips/uma_machdep.c
index 798e632..24baef0 100644
--- a/sys/mips/mips/uma_machdep.c
+++ b/sys/mips/mips/uma_machdep.c
@@ -48,11 +48,7 @@ uma_small_alloc(uma_zone_t zone, int bytes, u_int8_t *flags,
int wait)
void *va;
*flags = UMA_SLAB_PRIV;
-
- if ((wait & (M_NOWAIT|M_USE_RESERVE)) == M_NOWAIT)
- pflags = VM_ALLOC_INTERRUPT;
- else
- pflags = VM_ALLOC_SYSTEM;
+ pflags = m2vm_flags(wait, 0);
for (;;) {
m = pmap_alloc_direct_page(0, pflags);
diff --git a/sys/powerpc/aim/mmu_oea64.c b/sys/powerpc/aim/mmu_oea64.c
index a491680..3e320b9 100644
--- a/sys/powerpc/aim/mmu_oea64.c
+++ b/sys/powerpc/aim/mmu_oea64.c
@@ -1369,12 +1369,7 @@ moea64_uma_page_alloc(uma_zone_t zone, int bytes,
u_int8_t *flags, int wait)
*flags = UMA_SLAB_PRIV;
needed_lock = !PMAP_LOCKED(kernel_pmap);
- if ((wait & (M_NOWAIT|M_USE_RESERVE)) == M_NOWAIT)
- pflags = VM_ALLOC_INTERRUPT | VM_ALLOC_WIRED;
- else
- pflags = VM_ALLOC_SYSTEM | VM_ALLOC_WIRED;
- if (wait & M_ZERO)
- pflags |= VM_ALLOC_ZERO;
+ pflags = m2vm_flags(wait, VM_ALLOC_WIRED);
for (;;) {
m = vm_page_alloc(NULL, 0, pflags | VM_ALLOC_NOOBJ);
diff --git a/sys/powerpc/aim/slb.c b/sys/powerpc/aim/slb.c
index 162c7fb..3882bfa 100644
--- a/sys/powerpc/aim/slb.c
+++ b/sys/powerpc/aim/slb.c
@@ -483,12 +483,7 @@ slb_uma_real_alloc(uma_zone_t zone, int bytes, u_int8_t
*flags, int wait)
realmax = platform_real_maxaddr();
*flags = UMA_SLAB_PRIV;
- if ((wait & (M_NOWAIT | M_USE_RESERVE)) == M_NOWAIT)
- pflags = VM_ALLOC_INTERRUPT | VM_ALLOC_NOOBJ | VM_ALLOC_WIRED;
- else
- pflags = VM_ALLOC_SYSTEM | VM_ALLOC_NOOBJ | VM_ALLOC_WIRED;
- if (wait & M_ZERO)
- pflags |= VM_ALLOC_ZERO;
+ pflags = m2vm_flags(wait, VM_ALLOC_NOOBJ | VM_ALLOC_WIRED);
for (;;) {
m = vm_page_alloc_contig(NULL, 0, pflags, 1, 0, realmax,
diff --git a/sys/powerpc/aim/uma_machdep.c b/sys/powerpc/aim/uma_machdep.c
index 39deb43..23a333f 100644
--- a/sys/powerpc/aim/uma_machdep.c
+++ b/sys/powerpc/aim/uma_machdep.c
@@ -56,12 +56,7 @@ uma_small_alloc(uma_zone_t zone, int bytes, u_int8_t *flags,
int wait)
int pflags;
*flags = UMA_SLAB_PRIV;
- if ((wait & (M_NOWAIT|M_USE_RESERVE)) == M_NOWAIT)
- pflags = VM_ALLOC_INTERRUPT | VM_ALLOC_WIRED;
- else
- pflags = VM_ALLOC_SYSTEM | VM_ALLOC_WIRED;
- if (wait & M_ZERO)
- pflags |= VM_ALLOC_ZERO;
+ pflags = m2vm_flags(wait, VM_ALLOC_WIRED);
for (;;) {
m = vm_page_alloc(NULL, 0, pflags | VM_ALLOC_NOOBJ);
diff --git a/sys/sparc64/sparc64/vm_machdep.c b/sys/sparc64/sparc64/vm_machdep.c
index cdb94c7..573ab3a 100644
--- a/sys/sparc64/sparc64/vm_machdep.c
+++ b/sys/sparc64/sparc64/vm_machdep.c
@@ -501,14 +501,7 @@ uma_small_alloc(uma_zone_t zone, int bytes, u_int8_t
*flags, int wait)
PMAP_STATS_INC(uma_nsmall_alloc);
*flags = UMA_SLAB_PRIV;
-
- if ((wait & (M_NOWAIT|M_USE_RESERVE)) == M_NOWAIT)
- pflags = VM_ALLOC_INTERRUPT | VM_ALLOC_WIRED;
- else
- pflags = VM_ALLOC_SYSTEM | VM_ALLOC_WIRED;
-
- if (wait & M_ZERO)
- pflags |= VM_ALLOC_ZERO;
+ pflags = m2vm_flags(wait, VM_ALLOC_WIRED);
for (;;) {
m = vm_page_alloc(NULL, 0, pflags | VM_ALLOC_NOOBJ);
diff --git a/sys/vm/vm_kern.c b/sys/vm/vm_kern.c
index 46e7f1c..e4c3704 100644
--- a/sys/vm/vm_kern.c
+++ b/sys/vm/vm_kern.c
@@ -222,12 +222,7 @@ kmem_alloc_attr(vm_map_t map, vm_size_t size, int flags,
vm_paddr_t low,
vm_object_reference(object);
vm_map_insert(map, object, offset, addr, addr + size, VM_PROT_ALL,
VM_PROT_ALL, 0);
- if ((flags & (M_NOWAIT | M_USE_RESERVE)) == M_NOWAIT)
- pflags = VM_ALLOC_INTERRUPT | VM_ALLOC_NOBUSY;
- else
- pflags = VM_ALLOC_SYSTEM | VM_ALLOC_NOBUSY;
- if (flags & M_ZERO)
- pflags |= VM_ALLOC_ZERO;
+ pflags = m2vm_flags(flags, VM_ALLOC_NOBUSY);
VM_OBJECT_LOCK(object);
end_offset = offset + size;
for (; offset < end_offset; offset += PAGE_SIZE) {
@@ -296,14 +291,7 @@ kmem_alloc_contig(vm_map_t map, vm_size_t size, int flags,
vm_paddr_t low,
vm_object_reference(object);
vm_map_insert(map, object, offset, addr, addr + size, VM_PROT_ALL,
VM_PROT_ALL, 0);
- if ((flags & (M_NOWAIT | M_USE_RESERVE)) == M_NOWAIT)
- pflags = VM_ALLOC_INTERRUPT | VM_ALLOC_NOBUSY;
- else
- pflags = VM_ALLOC_SYSTEM | VM_ALLOC_NOBUSY;
- if (flags & M_ZERO)
- pflags |= VM_ALLOC_ZERO;
- if (flags & M_NODUMP)
- pflags |= VM_ALLOC_NODUMP;
+ pflags = m2vm_flags(flags, VM_ALLOC_NOBUSY);
VM_OBJECT_LOCK(object);
tries = 0;
retry:
@@ -487,11 +475,7 @@ kmem_back(vm_map_t map, vm_offset_t addr, vm_size_t size,
int flags)
entry->wired_count == 0 && (entry->eflags & MAP_ENTRY_IN_TRANSITION)
== 0, ("kmem_back: entry not found or misaligned"));
- if ((flags & (M_NOWAIT|M_USE_RESERVE)) == M_NOWAIT)
- pflags = VM_ALLOC_INTERRUPT | VM_ALLOC_WIRED;
- else
- pflags = VM_ALLOC_SYSTEM | VM_ALLOC_WIRED;
-
+ pflags = m2vm_flags(flags, VM_ALLOC_WIRED);
if (flags & M_ZERO)
pflags |= VM_ALLOC_ZERO;
if (flags & M_NODUMP)
diff --git a/sys/vm/vm_page.h b/sys/vm/vm_page.h
index 70b8416..0286a6d 100644
--- a/sys/vm/vm_page.h
+++ b/sys/vm/vm_page.h
@@ -344,6 +344,24 @@ extern struct mtx_padalign vm_page_queue_mtx;
#define VM_ALLOC_COUNT_SHIFT 16
#define VM_ALLOC_COUNT(count) ((count) << VM_ALLOC_COUNT_SHIFT)
+#ifdef M_NOWAIT
+static inline int
+m2vm_flags(int malloc_flags, int alloc_flags)
+{
+ int pflags;
+
+ if ((malloc_flags & (M_NOWAIT | M_USE_RESERVE)) == M_NOWAIT)
+ pflags = VM_ALLOC_SYSTEM | alloc_flags;
+ else
+ pflags = VM_ALLOC_INTERRUPT | alloc_flags;
+ if (malloc_flags & M_ZERO)
+ pflags |= VM_ALLOC_ZERO;
+ if (malloc_flags & M_NODUMP)
+ pflags |= VM_ALLOC_NODUMP;
+ return (pflags);
+}
+#endif
+
void vm_page_busy(vm_page_t m);
void vm_page_flash(vm_page_t m);
void vm_page_io_start(vm_page_t m);
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