[PATCH] dax: Fix ENOMEM handling in grab_mapping_entry()

[PATCH] dax: Fix ENOMEM handling in grab_mapping_entry()

[Jan Kara]

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 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.

Fixes: b15cd800682f ("dax: Convert page fault handlers to XArray")
Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/dax.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/dax.c b/fs/dax.c
index 62352cbcf0f4..da41f9363568 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -488,10 +488,11 @@ static void *grab_mapping_entry(struct xa_state *xas,
 		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);
 
-- 
2.26.2

Re: [PATCH] dax: Fix ENOMEM handling in grab_mapping_entry()

[Dan Williams]

Hi Jan,

I was out for a few days...

On Tue, Jun 22, 2021 at 9:00 AM Jan Kara <jack@suse.cz> wrote:
>
> 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 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.
>
> Fixes: b15cd800682f ("dax: Convert page fault handlers to XArray")
> Signed-off-by: Jan Kara <jack@suse.cz>

Looks good to me:

Reviewed-by: Dan Williams <dan.j.williams@intel.com>

I notice that Andrew already picked this up and I'm ok to let it
percolate up to Linus through -mm.