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

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Message-ID: <CAHbLzkrVoK-y4zc10+=0hDGZLi8+i73wSHciTUOWGDBsEcD0xw@mail.gmail.com>
Date: Fri, 15 Nov 2024 14:15:20 -0800
From: Yang Shi <shy828301@...il.com>
To: Kalesh Singh <kaleshsingh@...gle.com>
Cc: kernel-team@...roid.com, android-mm@...gle.com, 
	Andrew Morton <akpm@...ux-foundation.org>, Vlastimil Babka <vbabka@...e.cz>, 
	Yang Shi <yang@...amperecomputing.com>, Rik van Riel <riel@...riel.com>, 
	Ryan Roberts <ryan.roberts@....com>, Suren Baghdasaryan <surenb@...gle.com>, 
	Minchan Kim <minchan@...nel.org>, Hans Boehm <hboehm@...gle.com>, 
	Lokesh Gidra <lokeshgidra@...gle.com>, stable@...r.kernel.org, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Jann Horn <jannh@...gle.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: Respect mmap hint address when aligning for THP

On Fri, Nov 15, 2024 at 1:52 PM Kalesh Singh <kaleshsingh@...gle.com> 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]().
>
> 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.

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.

> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Yang Shi <yang@...amperecomputing.com>
> Cc: Rik van Riel <riel@...riel.com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: Suren Baghdasaryan <surenb@...gle.com>
> Cc: Minchan Kim <minchan@...nel.org>
> Cc: Hans Boehm <hboehm@...gle.com>
> Cc: Lokesh Gidra <lokeshgidra@...gle.com>
> Cc: <stable@...r.kernel.org>
> Fixes: efa7df3e3bb5 ("mm: align larger anonymous mappings on THP boundaries")
> Signed-off-by: Kalesh Singh <kaleshsingh@...gle.com>
> ---
>  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
>