Re: [RESEND PATCH] mm: align larger anonymous mappings on THP boundaries

[Yang Shi]

On Mon, Jan 22, 2024 at 3:37 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 20/01/2024 16:39, Matthew Wilcox wrote:
> > On Sat, Jan 20, 2024 at 12:04:27PM +0000, Ryan Roberts wrote:
> >> However, after this patch, each allocation is in its own VMA, and there is a 2M
> >> gap between each VMA. This causes 2 problems: 1) mmap becomes MUCH slower
> >> because there are so many VMAs to check to find a new 1G gap. 2) It fails once
> >> it hits the VMA limit (/proc/sys/vm/max_map_count). Hitting this limit then
> >> causes a subsequent calloc() to fail, which causes the test to fail.
> >>
> >> Looking at the code, I think the problem is that arm64 selects
> >> ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT. But __thp_get_unmapped_area() allocates
> >> len+2M then always aligns to the bottom of the discovered gap. That causes the
> >> 2M hole. As far as I can see, x86 allocates bottom up, so you don't get a hole.
> >
> > As a quick hack, perhaps
> > #ifdef ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
> > take-the-top-half
> > #else
> > current-take-bottom-half-code
> > #endif
> >
> > ?

Thanks for the suggestion. It makes sense to me. Doing the alignment
needs to take into account this.

>
> There is a general problem though that there is a trade-off between abutting
> VMAs, and aligning them to PMD boundaries. This patch has decided that in
> general the latter is preferable. The case I'm hitting is special though, in
> that both requirements could be achieved but currently are not.
>
> The below fixes it, but I feel like there should be some bitwise magic that
> would give the correct answer without the conditional - but my head is gone and
> I can't see it. Any thoughts?

Thanks Ryan for the patch. TBH I didn't see a bitwise magic without
the conditional either.

>
> Beyond this, though, there is also a latent bug where the offset provided to
> mmap() is carried all the way through to the get_unmapped_area()
> impelementation, even for MAP_ANONYMOUS - I'm pretty sure we should be
> force-zeroing it for MAP_ANONYMOUS? Certainly before this change, for arches
> that use the default get_unmapped_area(), any non-zero offset would not have
> been used. But this change starts using it, which is incorrect. That said, there
> are some arches that override the default get_unmapped_area() and do use the
> offset. So I'm not sure if this is a bug or a feature that user space can pass
> an arbitrary value to the implementation for anon memory??

Thanks for noticing this. If I read the code correctly, the pgoff used
by some arches to workaround VIPT caches, and it looks like it is for
shared mapping only (just checked arm and mips). And I believe
everybody assumes 0 should be used when doing anonymous mapping. The
offset should have nothing to do with seeking proper unmapped virtual
area. But the pgoff does make sense for file THP due to the alignment
requirements. I think it should be zero'ed for anonymous mappings,
like:

diff --git a/mm/mmap.c b/mm/mmap.c
index 2ff79b1d1564..a9ed353ce627 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1830,6 +1830,7 @@ get_unmapped_area(struct file *file, unsigned
long addr, unsigned long len,
                pgoff = 0;
                get_area = shmem_get_unmapped_area;
        } else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
+               pgoff = 0;
                /* Ensures that larger anonymous mappings are THP aligned. */
                get_area = thp_get_unmapped_area;
        }

>
> Finally, the second test failure I reported (ksm_tests) is actually caused by a
> bug in the test code, but provoked by this change. So I'll send out a fix for
> the test code separately.
>
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 4f542444a91f..68ac54117c77 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -632,7 +632,7 @@ static unsigned long __thp_get_unmapped_area(struct file *filp,
>  {
>         loff_t off_end = off + len;
>         loff_t off_align = round_up(off, size);
> -       unsigned long len_pad, ret;
> +       unsigned long len_pad, ret, off_sub;
>
>         if (off_end <= off_align || (off_end - off_align) < size)
>                 return 0;
> @@ -658,7 +658,13 @@ static unsigned long __thp_get_unmapped_area(struct file *filp,
>         if (ret == addr)
>                 return addr;
>
> -       ret += (off - ret) & (size - 1);
> +       off_sub = (off - ret) & (size - 1);
> +
> +       if (current->mm->get_unmapped_area == arch_get_unmapped_area_topdown &&
> +           !off_sub)
> +               return ret + size;
> +
> +       ret += off_sub;
>         return ret;
>  }

I didn't spot any problem, would you please come up with a formal patch?