Re: [PATCH] mm: Respect mmap hint address when aligning for THP

[Kalesh Singh]

On Mon, Nov 18, 2024 at 9:05 AM Yang Shi <shy828301@xxxxxxxxx> wrote:
>
> On Sun, Nov 17, 2024 at 3:12 AM Vlastimil Babka <vbabka@xxxxxxx> wrote:
> >
> > On 11/15/24 23:15, Yang Shi wrote:
> > > On Fri, Nov 15, 2024 at 1:52 PM Kalesh Singh <kaleshsingh@xxxxxxxxxx> wrote:
> > >>
> > >> Commit efa7df3e3bb5 ("mm: align larger anonymous mappings on THP
> > >> boundaries") updated __get_unmapped_area() to align the start address
> > >> for the VMA to a PMD boundary if CONFIG_TRANSPARENT_HUGEPAGE=y.
> > >>
> > >> It does this by effectively looking up a region that is of size,
> > >> request_size + PMD_SIZE, and aligning up the start to a PMD boundary.
> > >>
> > >> Commit 4ef9ad19e176 ("mm: huge_memory: don't force huge page alignment
> > >> on 32 bit") opted out of this for 32bit due to regressions in mmap base
> > >> randomization.
> > >>
> > >> Commit d4148aeab412 ("mm, mmap: limit THP alignment of anonymous
> > >> mappings to PMD-aligned sizes") restricted this to only mmap sizes that
> > >> are multiples of the PMD_SIZE due to reported regressions in some
> > >> performance benchmarks -- which seemed mostly due to the reduced spatial
> > >> locality of related mappings due to the forced PMD-alignment.
> > >>
> > >> Another unintended side effect has emerged: When a user specifies an mmap
> > >> hint address, the THP alignment logic modifies the behavior, potentially
> > >> ignoring the hint even if a sufficiently large gap exists at the requested
> > >> hint location.
> > >>
> > >> Example Scenario:
> > >>
> > >> Consider the following simplified virtual address (VA) space:
> > >>
> > >>     ...
> > >>
> > >>     0x200000-0x400000 --- VMA A
> > >>     0x400000-0x600000 --- Hole
> > >>     0x600000-0x800000 --- VMA B
> > >>
> > >>     ...
> > >>
> > >> A call to mmap() with hint=0x400000 and len=0x200000 behaves differently:
> > >>
> > >>   - Before THP alignment: The requested region (size 0x200000) fits into
> > >>     the gap at 0x400000, so the hint is respected.
> > >>
> > >>   - After alignment: The logic searches for a region of size
> > >>     0x400000 (len + PMD_SIZE) starting at 0x400000.
> > >>     This search fails due to the mapping at 0x600000 (VMA B), and the hint
> > >>     is ignored, falling back to arch_get_unmapped_area[_topdown]().
> >

Hi all, Thanks for the reviews.

> > Hmm looks like the search is not done in the optimal way regardless of
> > whether or not it ignores a hint - it should be able to find the hole, no?

It's not able to find the hole in the example case because the size we
are looking for is now
(requested size + padding len) which is larger than the hole we have
at the hint address.

> >
> > >> In general the hint is effectively ignored, if there is any
> > >> existing mapping in the below range:
> > >>
> > >>      [mmap_hint + mmap_size, mmap_hint + mmap_size + PMD_SIZE)
> > >>
> > >> This changes the semantics of mmap hint; from ""Respect the hint if a
> > >> sufficiently large gap exists at the requested location" to "Respect the
> > >> hint only if an additional PMD-sized gap exists beyond the requested size".
> > >>
> > >> This has performance implications for allocators that allocate their heap
> > >> using mmap but try to keep it "as contiguous as possible" by using the
> > >> end of the exisiting heap as the address hint. With the new behavior
> > >> it's more likely to get a much less contiguous heap, adding extra
> > >> fragmentation and performance overhead.
> > >>
> > >> To restore the expected behavior; don't use thp_get_unmapped_area_vmflags()
> > >> when the user provided a hint address.
> >
> > Agreed, the hint should take precendence.
> >
> > > Thanks for fixing it. I agree we should respect the hint address. But
> > > this patch actually just fixed anonymous mapping and the file mappings
> > > which don't support thp_get_unmapped_area(). So I think you should
> > > move the hint check to __thp_get_unmapped_area().
> > >
> > > And Vlastimil's fix d4148aeab412 ("mm, mmap: limit THP alignment of
> > > anonymous mappings to PMD-aligned sizes") should be moved to there too
> > > IMHO.
> >
> > This was brought up, but I didn't want to do it as part of the stable fix as
> > that would change even situations that Rik's change didn't.
> > If the mmap hint change is another stable hotfix, I wouldn't conflate it
> > either. But we can try it for further development. But careful about just
> > moving the code as-is, the file-based mappings are different than anonymous
> > memory and I believe file offsets matter:
> >
> > https://lore.kernel.org/all/9d7c73f6-1e1a-458b-93c6-3b44959022e0@xxxxxxx/
> >
> > https://lore.kernel.org/all/5f7a49e8-0416-4648-a704-a7a67e8cd894@xxxxxxx/
>

I see, so I think we should keep the check here to address the
immediate regression for anonymous mappings.

I believe what we would need to address this longer term is to have
vma_iter_lowest() [1] vma_iter_highest() [2] take into account the
alignment when doing the search. That way we don't need to inflate the
search size to facilitate the manual alignment after the fact. I
haven't looked too too deeply into this, maybe Liam has some ideas
about that?

[1] https://github.com/torvalds/linux/blob/v6.12-rc7/mm/vma.h#L420

[2] https://github.com/torvalds/linux/blob/v6.12-rc7/mm/vma.h#L426

> Did some research about the history of the code, I found this commit:
>
> 97d3d0f9a1cf ("mm/huge_memory.c: thp: fix conflict of above-47bit hint
> address and PMD alignment"), it tried to fix "the function would not
> try to allocate PMD-aligned area if *any* hint address specified."
> It was for file mapping back then since anonymous mapping THP
> alignment was not supported yet.
>
> But it seems like this patch somehow tried to do something reverse. It
> may not be correct either.
>

IIUC Kirill's patch is doing the right thing (mostly), i.e. it will
return the hint address (without any rounding to PMD alignment). The
case it doesn't handle is what I mentioned above, where we aren't able
to find the hole at the hint address in the first place because the
hole is smaller than (size + padding len)

> With Vlastimis's fix, we just try to make the address THP aligned for
> anonymous mapping when the size is PMD aligned. So we don't need to
> take into account the padding for anonymous mapping anymore.
>

We still need to have padding length because PMD alignment of the size
doesn't mean that the start address returned by the search will be PMD
aligned. Inherently those are only PAGE aligned.

Thanks,
Kalesh

> So IIUC we should do something like:
>
> @@ -1085,7 +1085,11 @@ static unsigned long
> __thp_get_unmapped_area(struct file *filp,
>         if (off_end <= off_align || (off_end - off_align) < size)
>                 return 0;
>
> -       len_pad = len + size;
> +       if (filp)
> +               len_pad = len + size;
> +       else
> +               len_pad = len;
> +
>         if (len_pad < len || (off + len_pad) < off)
>                 return 0;
>
> >
> > >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > >> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > >> Cc: Yang Shi <yang@xxxxxxxxxxxxxxxxxxxxxx>
> > >> Cc: Rik van Riel <riel@xxxxxxxxxxx>
> > >> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> > >> Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > >> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > >> Cc: Hans Boehm <hboehm@xxxxxxxxxx>
> > >> Cc: Lokesh Gidra <lokeshgidra@xxxxxxxxxx>
> > >> Cc: <stable@xxxxxxxxxxxxxxx>
> > >> Fixes: efa7df3e3bb5 ("mm: align larger anonymous mappings on THP boundaries")
> > >> Signed-off-by: Kalesh Singh <kaleshsingh@xxxxxxxxxx>
> >
> > Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
> >
> > >> ---
> > >>  mm/mmap.c | 1 +
> > >>  1 file changed, 1 insertion(+)
> > >>
> > >> diff --git a/mm/mmap.c b/mm/mmap.c
> > >> index 79d541f1502b..2f01f1a8e304 100644
> > >> --- a/mm/mmap.c
> > >> +++ b/mm/mmap.c
> > >> @@ -901,6 +901,7 @@ __get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
> > >>         if (get_area) {
> > >>                 addr = get_area(file, addr, len, pgoff, flags);
> > >>         } else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)
> > >> +                  && !addr /* no hint */
> > >>                    && IS_ALIGNED(len, PMD_SIZE)) {
> > >>                 /* Ensures that larger anonymous mappings are THP aligned. */
> > >>                 addr = thp_get_unmapped_area_vmflags(file, addr, len,
> > >>
> > >> base-commit: 2d5404caa8c7bb5c4e0435f94b28834ae5456623
> > >> --
> > >> 2.47.0.338.g60cca15819-goog
> > >>
> >