+ dax-fix-enomem-handling-in-grab_mapping_entry.patch added to -mm tree

+ dax-fix-enomem-handling-in-grab_mapping_entry.patch added to -mm tree

[akpm]

The patch titled
     Subject: dax: fix ENOMEM handling in grab_mapping_entry()
has been added to the -mm tree.  Its filename is
     dax-fix-enomem-handling-in-grab_mapping_entry.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/dax-fix-enomem-handling-in-grab_mapping_entry.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/dax-fix-enomem-handling-in-grab_mapping_entry.patch

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------------------------------------------------------
From: Jan Kara <jack@suse.cz>
Subject: dax: fix ENOMEM handling in grab_mapping_entry()

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: b15cd800682f ("dax: Convert page fault handlers to XArray")
Signed-off-by: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 fs/dax.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/fs/dax.c~dax-fix-enomem-handling-in-grab_mapping_entry
+++ a/fs/dax.c
@@ -488,10 +488,11 @@ static void *grab_mapping_entry(struct x
 		struct address_space *mapping, unsigned int order)
 {
 	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;
 
 retry:
+	pmd_downgrade = false;
 	xas_lock_irq(xas);
 	entry = get_unlocked_entry(xas, order);
 
_

Patches currently in -mm which might be from jack@suse.cz are

dax-fix-enomem-handling-in-grab_mapping_entry.patch
mm-fix-comments-mentioning-i_mutex.patch