From: Jianyun Gao dax_range_compare_iter() has two bugs in how it calls dax_iomap_direct_access() and uses the returned addresses: 1. The size argument passed to dax_iomap_direct_access() is ALIGN(pos + len, PAGE_SIZE), which is an absolute file offset rather than a length. This causes the underlying dax_direct_access() to request a massively inflated nr_pages via PHYS_PFN(size). 2. The addresses returned by dax_iomap_direct_access() are page-aligned starting addresses, but memcmp() compares from saddr/daddr directly without adding the page-in-page offset of pos1/pos2. When pos1/pos2 are not page-aligned, this compares the wrong data. Fix the size by computing ALIGN(len + offset_in_page(pos), PAGE_SIZE), matching the pattern used in dax_iomap_copy_around(). Fix the address by adding offset_in_page(pos1)/offset_in_page(pos2) to saddr/daddr in the memcmp() call. Fixes: 6f7db3894ae2 ("fsdax: dedup file range to use a compare function") Cc: Signed-off-by: Jianyun Gao --- Changes in v2: - Add offset_in_page(pos1)/offset_in_page(pos2) to the saddr/daddr arguments in memcmp() to fix the page-in-page offset issue pointed out in the v1 review. - Use offset_in_page() macro instead of (pos & ~PAGE_MASK) for consistency with the rest of the codebase. v1: https://lore.kernel.org/lkml/20261010071453.6028-1-jianyungao89@gmail.com/ fs/dax.c | 12 +++++++----- 1 file changed, 7 insertions(+), 5 deletions(-) diff --git a/fs/dax.c b/fs/dax.c index 6ba50142eeb2..b08d408418bd 100644 --- a/fs/dax.c +++ b/fs/dax.c @@ -2197,6 +2197,7 @@ static int dax_range_compare_iter(struct iomap_iter *it_src, const struct iomap *dmap = &it_dest->iomap; loff_t pos1 = it_src->pos, pos2 = it_dest->pos; void *saddr, *daddr; + size_t size1, size2; int id, ret; len = min(len, min(smap->length, dmap->length)); @@ -2212,17 +2213,18 @@ static int dax_range_compare_iter(struct iomap_iter *it_src, } id = dax_read_lock(); - ret = dax_iomap_direct_access(smap, pos1, ALIGN(pos1 + len, PAGE_SIZE), - &saddr, NULL); + size1 = ALIGN(len + offset_in_page(pos1), PAGE_SIZE); + ret = dax_iomap_direct_access(smap, pos1, size1, &saddr, NULL); if (ret < 0) goto out_unlock; - ret = dax_iomap_direct_access(dmap, pos2, ALIGN(pos2 + len, PAGE_SIZE), - &daddr, NULL); + size2 = ALIGN(len + offset_in_page(pos2), PAGE_SIZE); + ret = dax_iomap_direct_access(dmap, pos2, size2, &daddr, NULL); if (ret < 0) goto out_unlock; - *same = !memcmp(saddr, daddr, len); + *same = !memcmp(saddr + offset_in_page(pos1), + daddr + offset_in_page(pos2), len); if (!*same) len = 0; dax_read_unlock(id); -- 2.34.1