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dax: fix ENOMEM handling in grab_mapping_entry()

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[ Upstream commit 1a14e3779dd58c16b30e56558146e5cc850ba8b0 ]

grab_mapping_entry() has a bug in handling of ENOMEM condition.  Suppose
we have a PMD entry at index i which we are downgrading to a PTE entry.
grab_mapping_entry() will set pmd_downgrade to true, lock the entry, clear
the entry in xarray, and decrement mapping->nrpages.  The it will call:

	entry = dax_make_entry(pfn_to_pfn_t(0), flags);
	dax_lock_entry(xas, entry);

which inserts new PTE entry into xarray.  However this may fail allocating
the new node.  We handle this by:

	if (xas_nomem(xas, mapping_gfp_mask(mapping) & ~__GFP_HIGHMEM))
		goto retry;

however pmd_downgrade stays set to true even though 'entry' returned from
get_unlocked_entry() will be NULL now.  And we will go again through the
downgrade branch.  This is mostly harmless except that mapping->nrpages is
decremented again and we temporarily have an invalid entry stored in
xarray.  Fix the problem by setting pmd_downgrade to false each time we
lookup the entry we work with so that it matches the entry we found.

Link: https://lkml.kernel.org/r/20210622160015.18004-1-jack@suse.cz
Fixes: b15cd80068 ("dax: Convert page fault handlers to XArray")
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Jan Kara 2021-06-28 19:35:04 -07:00 committed by Greg Kroah-Hartman
parent c015295b28
commit c872674da7
1 changed files with 2 additions and 1 deletions
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fs/dax.c
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@ -488,10 +488,11 @@ static void *grab_mapping_entry(struct xa_state *xas,
struct address_space *mapping, unsigned int order) struct address_space *mapping, unsigned int order)
{ {
unsigned long index = xas->xa_index; unsigned long index = xas->xa_index;
bool pmd_downgrade = false; /* splitting PMD entry into PTE entries? */ bool pmd_downgrade; /* splitting PMD entry into PTE entries? */
void *entry; void *entry;
retry: retry:
pmd_downgrade = false;
xas_lock_irq(xas); xas_lock_irq(xas);
entry = get_unlocked_entry(xas, order); entry = get_unlocked_entry(xas, order);