Re: [PATCH mm-unstable 17/17] mm: Respect mmap hint before THP alignment if allocation is possible

[Kalesh Singh <kaleshsingh@google.com>]

On Mon, Dec 9, 2024 at 7:37 PM Yang Shi <shy828301@gmail.com> wrote:
>
> On Mon, Dec 9, 2024 at 6:45 PM Kalesh Singh <kaleshsingh@google.com> wrote:
> >
> > Commit 249608ee4713 ("mm: respect mmap hint address when aligning for THP")
> > fallsback to PAGE_SIZE alignment instead of THP alignment
> > for anonymous mapping as long as a hint address is provided by the user
> > -- even if we weren't able to allocate the unmapped area at the hint
> > address in the end.
> >
> > This was done to address the immediate regression in anonymous mappings
> > where the hint address were being ignored in some cases; due to commit
> > efa7df3e3bb5 ("mm: align larger anonymous mappings on THP boundaries").
> >
> > It was later pointed out that this issue also existed for file-backed
> > mappings from file systems that use thp_get_unmapped_area() for their
> > .get_unmapped_area() file operation.
> >
> > The same fix was not applied for file-backed mappings since it would
> > mean any mmap requests that provide a hint address would be only
> > PAGE_SIZE-aligned regardless of whether allocation was successful at
> > the hint address or not.
> >
> > Instead, use arch_mmap_hint() to first attempt allocation at the hint
> > address and fallback to THP alignment if that fails.
>
> Thanks for taking time to try to fix this.
>
> >
> > Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
> > ---
> >  mm/huge_memory.c | 15 ++++++++-------
> >  mm/mmap.c        |  1 -
> >  2 files changed, 8 insertions(+), 8 deletions(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 137abeda8602..f070c89dafc9 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -1097,6 +1097,14 @@ static unsigned long __thp_get_unmapped_area(struct file *filp,
> >         loff_t off_align = round_up(off, size);
> >         unsigned long len_pad, ret, off_sub;
> >
> > +       /*
> > +        * If allocation at the address hint succeeds; respect the hint and
> > +        * don't try to align to THP boundary.
> > +        */
> > +       addr = arch_mmap_hint(filp, addr, len, off, flags);
> > +       if (addr)
> > +               return addr;
> > +

Hi Yang,

Thanks for the comments.

>
> IIUC, arch_mmap_hint() will be called in arch_get_unmapped_area() and
> arch_get_unmapped_area_topdown() again. So we will actually look up
> maple tree twice. It sounds like the second hint address search is
> pointless. You should be able to set addr to 0 before calling
> mm_get_unmapped_area_vmflags() in order to skip the second hint
> address search.

You are right that it would call into arch_mmap_hint() twice but it
only attempts the lookup once since on the second attempt addr == 0.

Thanks,
Kalesh
>
> >         if (!IS_ENABLED(CONFIG_64BIT) || in_compat_syscall())
> >                 return 0;
> >
> > @@ -1117,13 +1125,6 @@ static unsigned long __thp_get_unmapped_area(struct file *filp,
> >         if (IS_ERR_VALUE(ret))
> >                 return 0;
> >
> > -       /*
> > -        * Do not try to align to THP boundary if allocation at the address
> > -        * hint succeeds.
> > -        */
> > -       if (ret == addr)
> > -               return addr;
> > -
> >         off_sub = (off - ret) & (size - 1);
> >
> >         if (test_bit(MMF_TOPDOWN, &current->mm->flags) && !off_sub)
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 59bf7d127aa1..6bfeec80152a 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -807,7 +807,6 @@ __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) && !file
> > -                  && !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,
> > --
> > 2.47.0.338.g60cca15819-goog
> >
> >