On 16.03.22 10:37, Dr. David Alan Gilbert wrote: > * Peter Maydell ([email protected]) wrote: >> On Wed, 16 Mar 2022 at 07:53, David Hildenbrand <[email protected]> wrote: >>> >>> On 16.03.22 05:04, Andrew Deason wrote: >>>> We have a thin wrapper around madvise, called qemu_madvise, which >>>> provides consistent behavior for the !CONFIG_MADVISE case, and works >>>> around some platform-specific quirks (some platforms only provide >>>> posix_madvise, and some don't offer all 'advise' types). This specific >>>> caller of madvise has never used it, tracing back to its original >>>> introduction in commit e0b266f01dd2 ("migration_completion: Take >>>> current state"). >>>> >>>> Call qemu_madvise here, to follow the same logic as all of our other >>>> madvise callers. This slightly changes the behavior for >>>> !CONFIG_MADVISE (EINVAL instead of ENOSYS, and a slightly different >>>> error message), but this is now more consistent with other callers >>>> that use qemu_madvise. >>>> >>>> Signed-off-by: Andrew Deason <[email protected]> >>>> --- >>>> Looking at the history of commits that touch this madvise() call, it >>>> doesn't _look_ like there's any reason to be directly calling madvise vs >>>> qemu_advise (I don't see anything mentioned), but I'm not sure. >>>> >>>> softmmu/physmem.c | 12 ++---------- >>>> 1 file changed, 2 insertions(+), 10 deletions(-) >>>> >>>> diff --git a/softmmu/physmem.c b/softmmu/physmem.c >>>> index 43ae70fbe2..900c692b5e 100644 >>>> --- a/softmmu/physmem.c >>>> +++ b/softmmu/physmem.c >>>> @@ -3584,40 +3584,32 @@ int ram_block_discard_range(RAMBlock *rb, uint64_t >>>> start, size_t length) >>>> rb->idstr, start, length, ret); >>>> goto err; >>>> #endif >>>> } >>>> if (need_madvise) { >>>> /* For normal RAM this causes it to be unmapped, >>>> * for shared memory it causes the local mapping to disappear >>>> * and to fall back on the file contents (which we just >>>> * fallocate'd away). >>>> */ >>>> -#if defined(CONFIG_MADVISE) >>>> if (qemu_ram_is_shared(rb) && rb->fd < 0) { >>>> - ret = madvise(host_startaddr, length, QEMU_MADV_REMOVE); >>>> + ret = qemu_madvise(host_startaddr, length, >>>> QEMU_MADV_REMOVE); >>>> } else { >>>> - ret = madvise(host_startaddr, length, QEMU_MADV_DONTNEED); >>>> + ret = qemu_madvise(host_startaddr, length, >>>> QEMU_MADV_DONTNEED); >>> >>> posix_madvise(QEMU_MADV_DONTNEED) has completely different semantics >>> then madvise() -- it's not a discard that we need here. >>> >>> So ram_block_discard_range() would now succeed in environments (BSD?) >>> where it's supposed to fail. >>> >>> So AFAIKs this isn't sane. >> >> But CONFIG_MADVISE just means "host has madvise()"; it doesn't imply >> "this is a Linux madvise() with MADV_DONTNEED". Solaris madvise() >> doesn't seem to have MADV_DONTNEED at all; a quick look at the >> FreeBSD manpage suggests its madvise MADV_DONTNEED is identical >> to its posix_madvise MADV_DONTNEED. >> >> If we need "specifically Linux MADV_DONTNEED semantics" maybe we >> should define a QEMU_MADV_LINUX_DONTNEED which either (a) does the >> right thing or (b) fails, and use qemu_madvise() regardless. >> >> Certainly the current code is pretty fragile to being changed by >> people who don't understand the undocumented subtlety behind >> the use of a direct madvise() call here. > > Yeh and I'm not sure I can remembe rall the subtleties; there's a big > hairy set of ifdef's in include/qemu/madvise.h that makes > sure we always have the definition of QEMU_MADV_REMOVE/DONTNEED > even on platforms that might not define it themselves. > > But I think this code is used for things with different degrees > of care about the semantics; e.g. 'balloon' just cares that > it frees memory up and doesn't care about the detailed semantics > that much; so it's probably fine with that. > Postcopy is much more touchy, but then it's only going to be > calling this on Linux anyway (because of the userfault dependency).
MADV_DONTNEED/MADV_REMOVE only provides discard semantics on Linux IIRC -- and that's what we want to achieve: ram_block_discard_range() So I agree with Peter that we might want to make this more explicit. -- Thanks, David / dhildenb
