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    SubjectRe: [PATCH] mm for fs: add truncate_pagecache_range
    On Fri, 23 Mar 2012, Andrew Morton wrote:
    > On Fri, 23 Mar 2012 14:14:54 -0700 (PDT)
    > Hugh Dickins <hughd@google.com> wrote:
    > > On Fri, 23 Mar 2012, Andrew Morton wrote:
    > > >
    > > > --- a/mm/truncate.c~mm-for-fs-add-truncate_pagecache_range-fix
    > > > +++ a/mm/truncate.c
    > > > @@ -639,6 +639,9 @@ int vmtruncate_range(struct inode *inode
    > > > * with on-disk format, and the filesystem would not have to deal with
    > > > * situations such as writepage being called for a page that has already
    > > > * had its underlying blocks deallocated.
    > > > + *
    > > > + * Must be called with inode->i_mapping->i_mutex held.
    > >
    > > You catch me offguard: I forget whether that's an absolute requirement or
    > > just commonly the case. What do the other interfaces in truncate.c say ?-)
    >
    > i_mutex is generally required, to stabilise i_size.

    Sorry for being quarrelsome, but I do want to Nak your followup "fix".

    Building a test kernel quickly told me that inode->i_mapping->i_mutex
    doesn't exist, of course it's inode->i_mutex.

    Then running the test kernel quickly told me that neither ext4 nor xfs
    (I didn't try ocfs2) holds inode->i_mutex where holepunching calls
    truncate_inode_pages_range().

    Now, there might or might not be reasons why ext4 or xfs ought to hold
    i_mutex there for its own consistency, but it's beyond me to determine
    that: let's assume they're correct without evidence to the contrary.

    Stabilizing i_size is not a reason: holepunching does not affect i_size
    and is not affected by i_size (okay, ext4 still has the bug I reported
    a couple of months ago, whereby its holepunching stops at i_size,
    forgetting blocks fallocated beyond; but no doubt that will get fixed).

    And nothing that truncate_pagecache_range() does needs i_mutex:
    neither the unmap_mapping_range() nor the truncate_inode_pages_range()
    needs i_mutex. A year ago, yes, Miklos showed how unmap_mapping_range()
    was relying on mutex serialization, and added an additional mutex for
    that, which Peter was able to remove once he mutified i_mmap_lock.

    truncate_pagecache_range() is just a drop-in replacement for
    truncate_inode_pages_range(), and has no different locking needs.

    Hugh


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