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

[Yang Shi]

On Tue, Jan 23, 2024 at 9:26 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 23/01/2024 17:14, Yang Shi wrote:
> > On Tue, Jan 23, 2024 at 1:41 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
> >>
> >> On 22/01/2024 19:43, Yang Shi wrote:
> >>> 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;
> >>>         }
> >>
> >> I think it would be cleaner to just zero pgoff if file==NULL, then it covers the
> >> shared case, the THP case, and the non-THP case properly. I'll prepare a
> >> separate patch for this.
> >
> > IIUC I don't think this is ok for those arches which have to
> > workaround VIPT cache since MAP_ANONYMOUS | MAP_SHARED with NULL file
> > pointer is a common case for creating tmpfs mapping. For example,
> > arm's arch_get_unmapped_area() has:
> >
> > if (aliasing)
> >         do_align = filp || (flags & MAP_SHARED);
> >
> > The pgoff is needed if do_align is true. So we should just zero pgoff
> > iff !file && !MAP_SHARED like what my patch does, we can move the
> > zeroing to a better place.
>
> We crossed streams - I sent out the patch just as you sent this. My patch is
> implemented as I proposed.

We crossed again :-)

>
> I'm not sure I agree with what you are saying. The mmap man page says this:
>
>   The  contents  of  a file mapping (as opposed to an anonymous mapping; see
>   MAP_ANONYMOUS below), are initialized using length bytes starting at offset
>   offset in the file (or other object) referred to by the file descriptor fd.
>
> So that implies offset is only relavent when a file is provided. It then goes on
> to say:
>
>   MAP_ANONYMOUS
>   The mapping is not backed by any file; its contents are initialized to zero.
>   The fd argument is ignored; however, some implementations require fd to be -1
>   if MAP_ANONYMOUS (or MAP_ANON) is specified, and portable applications should
>   ensure this. The offset argument should be zero.
>
> So users are expected to pass offset=0 when mapping anon memory, for both shared
> and private cases.
>
> Infact, in the line above where you made your proposed change, pgoff is also
> being zeroed for the (!file && (flags & MAP_SHARED)) case.

Yeah, rethinking led me to the same conclusion.

>
>
> >
> >>
> >>
> >>>
> >>>>
> >>>> 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?
> >>
> >> Yeah, I'll aim to post today.
> >
> > Thanks!
> >
> >>
> >>
>