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    Subject[PATCH 5.12 344/700] dax: fix ENOMEM handling in grab_mapping_entry()
    Date
    From: Jan Kara <jack@suse.cz>

    [ 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: b15cd800682f ("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>
    ---
    fs/dax.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    diff --git a/fs/dax.c b/fs/dax.c
    index df5485b4bddf..d5d7b9393bca 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);

    --
    2.30.2


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