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