mm: respect mmap hint address when aligning for THP

JIRA: https://issues.redhat.com/browse/RHEL-104908
Conflicts:
  Minor context difference due to bringing in 34d7cf6 ("mm: don't
  try THP alignment for FS without get_unmapped_area") out of order.

This patch is a backport of the following upstream commit:
commit 249608e
Author: Kalesh Singh <[email protected]>
Date:   Mon Nov 18 13:46:48 2024 -0800

    mm: respect mmap hint address when aligning for THP

    Commit efa7df3 ("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 4ef9ad1 ("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 d4148ae ("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, for
    anonymous mappings.

    Note: As Yang Shi pointed out: the issue still remains for filesystems
    which are using thp_get_unmapped_area() for their get_unmapped_area() op.
    It is unclear what worklaods will regress for if we ignore THP alignment
    when the hint address is provided for such file backed mappings -- so this
    fix will be handled separately.

    Link: https://lkml.kernel.org/r/[email protected]
    Fixes: efa7df3 ("mm: align larger anonymous mappings on THP boundaries")
    Signed-off-by: Kalesh Singh <[email protected]>
    Reviewed-by: Rik van Riel <[email protected]>
    Reviewed-by: Vlastimil Babka <[email protected]>
    Reviewed-by: David Hildenbrand <[email protected]>
    Cc: Kefeng Wang <[email protected]>
    Cc: Vlastimil Babka <[email protected]>
    Cc: Yang Shi <[email protected]>
    Cc: Rik van Riel <[email protected]>
    Cc: Ryan Roberts <[email protected]>
    Cc: Suren Baghdasaryan <[email protected]>
    Cc: Minchan Kim <[email protected]>
    Cc: Hans Boehm <[email protected]>
    Cc: Lokesh Gidra <[email protected]>
    Cc: <[email protected]>
    Signed-off-by: Andrew Morton <[email protected]>

Signed-off-by: Audra Mitchell <[email protected]>

Author: Audra Mitchell

mm/mmap.c

@@ -1868,6 +1868,7 @@ __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) && !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,