On Tue, Jan 23, 2024 at 9:14 AM Ryan Roberts ryan.roberts@arm.com wrote:
The addition of commit efa7df3e3bb5 ("mm: align larger anonymous mappings on THP boundaries") caused the "virtual_address_range" mm selftest to start failing on arm64. Let's fix that regression.
There were 2 visible problems when running the test; 1) it takes much longer to execute, and 2) the test fails. Both are related:
The (first part of the) test allocates as many 1GB anonymous blocks as it can in the low 256TB of address space, passing NULL as the addr hint to mmap. Before the faulty patch, all allocations were abutted and contained in a single, merged VMA. However, after this patch, each allocation is in its own VMA, and there is a 2M gap between each VMA. This causes the 2 problems in the test: 1) mmap becomes MUCH slower because there are so many VMAs to check to find a new 1G gap. 2) mmap fails once it hits the VMA limit (/proc/sys/vm/max_map_count). Hitting this limit then causes a subsequent calloc() to fail, which causes the test to fail.
The problem is that arm64 (unlike x86) selects ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT. But __thp_get_unmapped_area() allocates len+2M then always aligns to the bottom of the discovered gap. That causes the 2M hole.
Fix this by detecting cases where we can still achive the alignment goal when moved to the top of the allocated area, if configured to prefer top-down allocation.
While we are at it, fix thp_get_unmapped_area's use of pgoff, which should always be zero for anonymous mappings. Prior to the faulty change, while it was possible for user space to pass in pgoff!=0, the old mm->get_unmapped_area() handler would not use it. thp_get_unmapped_area() does use it, so let's explicitly zero it before calling the handler. This should also be the correct behavior for arches that define their own get_unmapped_area() handler.
Fixes: efa7df3e3bb5 ("mm: align larger anonymous mappings on THP boundaries") Closes: https://lore.kernel.org/linux-mm/1e8f5ac7-54ce-433a-ae53-81522b2320e1@arm.co... Cc: stable@vger.kernel.org Signed-off-by: Ryan Roberts ryan.roberts@arm.com
Thanks for debugging this. Looks good to me. Reviewed-by: Yang Shi shy828301@gmail.com
Applies on top of v6.8-rc1. Would be good to get this into the next -rc.
This may have a conflict with my fix (" mm: huge_memory: don't force huge page alignment on 32 bit") which is on mm-unstable now.
Thanks, Ryan
mm/huge_memory.c | 10 ++++++++-- mm/mmap.c | 6 ++++-- 2 files changed, 12 insertions(+), 4 deletions(-)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 94ef5c02b459..8c66f88e71e9 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -809,7 +809,7 @@ static unsigned long __thp_get_unmapped_area(struct file *filp, { loff_t off_end = off + len; loff_t off_align = round_up(off, size);
unsigned long len_pad, ret;
unsigned long len_pad, ret, off_sub; if (off_end <= off_align || (off_end - off_align) < size) return 0;@@ -835,7 +835,13 @@ static unsigned long __thp_get_unmapped_area(struct file *filp, if (ret == addr) return addr;
ret += (off - ret) & (size - 1);
off_sub = (off - ret) & (size - 1);if (current->mm->get_unmapped_area == arch_get_unmapped_area_topdown &&!off_sub)return ret + size;ret += off_sub; return ret;}
diff --git a/mm/mmap.c b/mm/mmap.c index b78e83d351d2..d89770eaab6b 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -1825,15 +1825,17 @@ get_unmapped_area(struct file *file, unsigned long addr, unsigned long len, /* * mmap_region() will call shmem_zero_setup() to create a file, * so use shmem's get_unmapped_area in case it can be huge.
* do_mmap() will clear pgoff, so match alignment. */pgoff = 0; get_area = shmem_get_unmapped_area; } else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) { /* Ensures that larger anonymous mappings are THP aligned. */ get_area = thp_get_unmapped_area; }
/* Always treat pgoff as zero for anonymous memory. */if (!file)pgoff = 0;addr = get_area(file, addr, len, pgoff, flags); if (IS_ERR_VALUE(addr)) return addr;-- 2.25.1