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

[Yang Shi <shy828301@gmail.com>]

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;
> +

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.

>         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
>
>