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swap: __swap_duplicate check bad swap entry

PATCH Hugh Dickins 1 participant 1 message Mar 19, 2013 1 message not in the archive

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Hugh Dickins

author Acked-by
#1 lore raw
Quoted 11 lines · Shaohua Li

In swapin_readahead(), read_swap_cache_async() can read a bad swap entry, because we don't check if readahead swap entry is bad. This doesn't break anything but such swapin page is wasteful and can only be freed at page reclaim. We avoid read such swap entry.

And next patch will mark a swap entry bad temporarily for discard. Without this patch, swap entry count will be messed.

Thanks Hugh to inspire swapin_readahead could use bad swap entry.

Signed-off-by Shaohua Li <[email protected]>
Acked-by Hugh Dickins <[email protected]>

Personally, I'd have merged this one into the next, or added it just after the next - it can be easier to explain a bug once it's already there, and okay to fix up after, just so long as it's very unlikely to interfere with anybody's future bisection. But maybe you and I just prefer to tell stories in a different way: no need for you to reorder.

I applied your v3 series shortly before 3.9-rc1, and ran it under load for a week. With this addition to your previous, I no longer saw any "Unused swap" or VM_BUG_ON(error == -EEXIST) issues, it all ran fine.

With one exception: swapoff occasionally failed, and it was another such SWAP_MAP_BAD issue. At the bottom I've appended the patch I was using to fix that (which of course only makes sense after your next).

Maybe you'd like to merge that into this 2/4, and exchange 2/4 and 3/4, or maybe you'd just like to merge it into your 3/4, or maybe you'd prefer to keep it as a separate fixup following 3/4: up to you, I'm not hung up the the ownership of it. (I never experimented without the SWP_WRITEOK part of it: I'm not sure how necessary that part is, but I feel safer with it in.)

Quoted 10 lines
---
 mm/swapfile.c |    5 +++++
 1 file changed, 5 insertions(+)

Index: linux/mm/swapfile.c
===================================================================

mm/swapfile.c +0 -0
@@ -2374,6 +2374,11 @@ static int __swap_duplicate(swp_entry_t
 		goto unlock_out;

mm/swapfile.c static int __swap_duplicate(swp_entry_t · line 2384

Quoted 10 lines
mm/swapfile.c +5 -0
 
 	count = p->swap_map[offset];
+	if (unlikely(swap_count(count) == SWAP_MAP_BAD)) {
+		err = -ENOENT;
+		goto unlock_out;
+	}
+
 	has_cache = count & SWAP_HAS_CACHE;
 	count &= ~SWAP_HAS_CACHE;
 	err = 0;

[PATCH] swap: fix swapoff ENOMEMs from discard

swapoff was sometimes failing with "Cannot allocate memory", coming from try_to_unuse()'s -ENOMEM: it needs to allow for swap_duplicate() failing on a free entry temporarily SWAP_MAP_BAD while being discarded.

We should use ACCESS_ONCE() there, and whenever accessing swap_map locklessly; but rather than peppering it throughout try_to_unuse(), just declare *swap_map with volatile.

try_to_unuse() is accustomed to *swap_map going down racily, but not necessarily to it jumping up from 0 to SWAP_MAP_BAD: we'll be safer to prevent that transition once SWP_WRITEOK is switched off, when it's a waste of time to issue discards anyway (swapon can do a whole discard).

Signed-off-by Hugh Dickins <[email protected]>

---

 mm/swapfile.c |   11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)
mm/swapfile.c +6 -5
@@ -199,8 +199,8 @@ static void discard_swap_cluster(struct
 static int swap_cluster_check_discard(struct swap_info_struct *si,
 		unsigned int idx)
 {
-
-	if (!(si->flags & SWP_DISCARDABLE))
+	if ((si->flags & (SWP_WRITEOK | SWP_DISCARDABLE)) !=
+			 (SWP_WRITEOK | SWP_DISCARDABLE))
 		return 0;
 	/*
 	 * If scan_swap_map() can't find a free cluster, it will check

@@ -1223,7 +1223,7 @@ static unsigned int find_next_to_unuse(s
 			else
 				continue;
 		}
-		count = si->swap_map[i];
+		count = ACCESS_ONCE(si->swap_map[i]);
 		if (count && swap_count(count) != SWAP_MAP_BAD)
 			break;
 	}

@@ -1243,7 +1243,7 @@ int try_to_unuse(unsigned int type, bool
 {
 	struct swap_info_struct *si = swap_info[type];
 	struct mm_struct *start_mm;
-	unsigned char *swap_map;
+	volatile unsigned char *swap_map;	/* ACCESS_ONCE throughout */
 	unsigned char swcount;
 	struct page *page;
 	swp_entry_t entry;

@@ -1294,7 +1294,8 @@ int try_to_unuse(unsigned int type, bool
 			 * reused since sys_swapoff() already disabled
 			 * allocation from here, or alloc_page() failed.
 			 */
-			if (!*swap_map)
+			swcount = *swap_map;
+			if (!swcount || swcount == SWAP_MAP_BAD)
 				continue;
 			retval = -ENOMEM;
 			break;

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