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authorHugh Dickins <hughd@google.com>2012-06-15 17:55:50 -0700
committerBen Hutchings <ben@decadent.org.uk>2012-06-19 23:18:28 +0100
commite0254df58aa9a0eede8b7f8dc1cb453e8f1e6b5c (patch)
tree4f4cec3ea39a9dd02bf0679320d50bc2455f067c
parent7e86185a3e138974ce88fe75baf3410addafc334 (diff)
swap: fix shmem swapping when more than 8 areas
commit 9b15b817f3d62409290fd56fe3cbb076a931bb0a upstream. Minchan Kim reports that when a system has many swap areas, and tmpfs swaps out to the ninth or more, shmem_getpage_gfp()'s attempts to read back the page cannot locate it, and the read fails with -ENOMEM. Whoops. Yes, I blindly followed read_swap_header()'s pte_to_swp_entry( swp_entry_to_pte()) technique for determining maximum usable swap offset, without stopping to realize that that actually depends upon the pte swap encoding shifting swap offset to the higher bits and truncating it there. Whereas our radix_tree swap encoding leaves offset in the lower bits: it's swap "type" (that is, index of swap area) that was truncated. Fix it by reducing the SWP_TYPE_SHIFT() in swapops.h, and removing the broken radix_to_swp_entry(swp_to_radix_entry()) from read_swap_header(). This does not reduce the usable size of a swap area any further, it leaves it as claimed when making the original commit: no change from 3.0 on x86_64, nor on i386 without PAE; but 3.0's 512GB is reduced to 128GB per swapfile on i386 with PAE. It's not a change I would have risked five years ago, but with x86_64 supported for ten years, I believe it's appropriate now. Hmm, and what if some architecture implements its swap pte with offset encoded below type? That would equally break the maximum usable swap offset check. Happily, they all follow the same tradition of encoding offset above type, but I'll prepare a check on that for next. Reported-and-Reviewed-and-Tested-by: Minchan Kim <minchan@kernel.org> Signed-off-by: Hugh Dickins <hughd@google.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
-rw-r--r--include/linux/swapops.h8
-rw-r--r--mm/swapfile.c12
2 files changed, 9 insertions, 11 deletions
diff --git a/include/linux/swapops.h b/include/linux/swapops.h
index 2189d3ffc85d..d69556075b09 100644
--- a/include/linux/swapops.h
+++ b/include/linux/swapops.h
@@ -8,13 +8,15 @@
* get good packing density in that tree, so the index should be dense in
* the low-order bits.
*
- * We arrange the `type' and `offset' fields so that `type' is at the five
+ * We arrange the `type' and `offset' fields so that `type' is at the seven
* high-order bits of the swp_entry_t and `offset' is right-aligned in the
- * remaining bits.
+ * remaining bits. Although `type' itself needs only five bits, we allow for
+ * shmem/tmpfs to shift it all up a further two bits: see swp_to_radix_entry().
*
* swp_entry_t's are *never* stored anywhere in their arch-dependent format.
*/
-#define SWP_TYPE_SHIFT(e) (sizeof(e.val) * 8 - MAX_SWAPFILES_SHIFT)
+#define SWP_TYPE_SHIFT(e) ((sizeof(e.val) * 8) - \
+ (MAX_SWAPFILES_SHIFT + RADIX_TREE_EXCEPTIONAL_SHIFT))
#define SWP_OFFSET_MASK(e) ((1UL << SWP_TYPE_SHIFT(e)) - 1)
/*
diff --git a/mm/swapfile.c b/mm/swapfile.c
index d3955f2745e5..fad1830ddda5 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1921,24 +1921,20 @@ static unsigned long read_swap_header(struct swap_info_struct *p,
/*
* Find out how many pages are allowed for a single swap
- * device. There are three limiting factors: 1) the number
+ * device. There are two limiting factors: 1) the number
* of bits for the swap offset in the swp_entry_t type, and
* 2) the number of bits in the swap pte as defined by the
- * the different architectures, and 3) the number of free bits
- * in an exceptional radix_tree entry. In order to find the
+ * different architectures. In order to find the
* largest possible bit mask, a swap entry with swap type 0
* and swap offset ~0UL is created, encoded to a swap pte,
* decoded to a swp_entry_t again, and finally the swap
* offset is extracted. This will mask all the bits from
* the initial ~0UL mask that can't be encoded in either
* the swp_entry_t or the architecture definition of a
- * swap pte. Then the same is done for a radix_tree entry.
+ * swap pte.
*/
maxpages = swp_offset(pte_to_swp_entry(
- swp_entry_to_pte(swp_entry(0, ~0UL))));
- maxpages = swp_offset(radix_to_swp_entry(
- swp_to_radix_entry(swp_entry(0, maxpages)))) + 1;
-
+ swp_entry_to_pte(swp_entry(0, ~0UL)))) + 1;
if (maxpages > swap_header->info.last_page) {
maxpages = swap_header->info.last_page + 1;
/* p->max is an unsigned int: don't overflow it */