Peter Maydell <peter.mayd...@linaro.org> writes:
> On Tue, 30 Jun 2020 at 11:36, Alex Bennée <alex.ben...@linaro.org> wrote: >> >> Given we assert the requested address matches what we asked we should >> also make that clear in the mmap flags. Otherwise we see failures in >> the GitLab environment for some currently unknown but allowable >> reason. > > Adding MAP_FIXED will mean that instead of failing if there's > something else already at that address, the kernel will now > silently blow that away in favour of the new mapping. Is > that definitely what we want here ? Hmm maybe not. But hey I just noticed that we have MAP_FIXED_NOREPLACE (since Linux 4.17) which says: This flag provides behavior that is similar to MAP_FIXED with respect to the addr enforcement, but differs in that MAP_FIXED_NOREPLACE never clobbers a preexisting mapped range. If the requested range would collide with an existing mapping, then this call fails with the error EEXIST. This flag can therefore be used as a way to atomically (with respect to other threads) attempt to map an address range: one thread will suc‐ ceed; all others will report failure. Note that older kernels which do not recognize the MAP_FIXED_NOREPLACE flag will typically (upon detecting a colli‐ sion with a preexisting mapping) fall back to a "non-MAP_FIXED" type of behavior: they will return an address that is different from the requested address. Therefore, backward-compatible software should check the returned address against the requested address. So maybe that is what we should do? Now you've pointed that out I wonder if we need to fix pgd_find_hole_fallback as well? > > thanks > -- PMM -- Alex Bennée
