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Subject[PATCH] swap writepages swizzled
tmpfs failed fsx+swapout tests after many hours, a page found zeroed.
Not a truncate problem, but mirror image of earlier truncate problems:
swap goes through mpage_writepages, which must therefore allow for a
sudden swizzle back to file identity.

Second time this caught us, so I've audited the tree for other places
which might be surprised by such swizzling. The only others I found
were (perhaps) in the parisc and sparc64 flush_dcache_page called
from do_generic_mapping_read on a looped tmpfs file which is also
mmapped; but that's a very marginal case, I wanted to understand it
better before making any edit, and now realize that hch's sendfile
in loop eliminates it (now go through do_shmem_file_read instead:
similar but crucially this locks the page when raising its count,
which is enough to keep vmscan from interfering).

Patch applies to 2.5.47, or 2.5.47-mm1 with offsets and fuzz.
Only the patch to mpage.c is required: I think it's worth adding
BUG_ON checks in __set_page_dirty_nobuffers and get_swap_bio,
just leave those out if you disagree; similarly, optional patch
to try_to_free_buffers, once upon a time swap came that way,
happily ever after it doesn't, so the test seems misleading.

Hugh

--- 2.5.47/fs/mpage.c Mon Nov 11 08:26:55 2002
+++ linux/fs/mpage.c Mon Nov 11 17:01:27 2002
@@ -587,12 +587,19 @@
page_cache_get(page);
write_unlock(&mapping->page_lock);

+ /*
+ * At this point we hold neither mapping->page_lock nor
+ * lock on the page itself: the page may be truncated or
+ * invalidated (changing page->mapping to NULL), or even
+ * swizzled back from swapper_space to tmpfs file mapping.
+ */
+
lock_page(page);

if (sync)
wait_on_page_writeback(page);

- if (page->mapping && !PageWriteback(page) &&
+ if (page->mapping == mapping && !PageWriteback(page) &&
test_clear_page_dirty(page)) {
if (writepage) {
ret = (*writepage)(page);
--- 2.5.47/mm/page-writeback.c Thu Oct 31 05:40:06 2002
+++ linux/mm/page-writeback.c Mon Nov 11 17:01:27 2002
@@ -613,6 +613,7 @@
if (mapping) {
write_lock(&mapping->page_lock);
if (page->mapping) { /* Race with truncate? */
+ BUG_ON(page->mapping != mapping);
if (!mapping->backing_dev_info->memory_backed)
inc_page_state(nr_dirty);
list_del(&page->list);
--- 2.5.47/mm/page_io.c Mon Oct 7 20:37:50 2002
+++ linux/mm/page_io.c Mon Nov 11 17:01:27 2002
@@ -30,6 +30,7 @@
struct swap_info_struct *sis;
swp_entry_t entry;

+ BUG_ON(!PageSwapCache(page));
entry.val = page->index;
sis = get_swap_info_struct(swp_type(entry));

--- 2.5.47/fs/buffer.c Mon Nov 11 08:26:55 2002
+++ linux/fs/buffer.c Mon Nov 11 17:01:27 2002
@@ -2496,7 +2496,7 @@

spin_lock(&mapping->private_lock);
ret = drop_buffers(page, &buffers_to_free);
- if (ret && !PageSwapCache(page)) {
+ if (ret) {
/*
* If the filesystem writes its buffers by hand (eg ext3)
* then we can have clean buffers against a dirty page. We
-
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