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    SubjectRe: ftruncate-mmap: pages are lost after writing to mmaped file.
    On Tue, Mar 24, 2009 at 3:32 AM, Andrew Morton
    <> wrote:
    > On Tue, 24 Mar 2009 18:44:21 +1100 Nick Piggin <> wrote:
    >> On Friday 20 March 2009 03:46:39 Jan Kara wrote:
    >> > On Fri 20-03-09 02:48:21, Nick Piggin wrote:
    >> > > Holding mapping->private_lock over the __set_page_dirty should
    >> > > fix it, although I guess you'd want to release it before calling
    >> > > __mark_inode_dirty so as not to put inode_lock under there. I
    >> > > have a patch for this if it sounds reasonable.
    >> >
    >> > Yes, that seems to be a bug - the function actually looked suspitious to
    >> > me yesterday but I somehow convinced myself that it's fine. Probably
    >> > because fsx-linux is single-threaded.
    >> After a whole lot of chasing my own tail in the VM and buffer layers,
    >> I think it is a problem in ext2 (and I haven't been able to reproduce
    >> with ext3 yet, which might lend weight to that, although as we have
    >> seen, it is very timing dependent).
    >> That would be slightly unfortunate because we still have Jan's ext3
    >> problem, and also another reported problem of corruption on ext3 (on
    >> brd driver).
    >> Anyway, when I have reproduced the problem with the test case, the
    >> "lost" writes are all reported to be holes. Unfortunately, that doesn't
    >> point straight to the filesystem, because ext2 allocates blocks in this
    >> case at writeout time, so if dirty bits are getting lost, then it would
    >> be normal to see holes.
    >> I then put in a whole lot of extra infrastructure to track metadata about
    >> each struct page (when it was last written out, when it last had the number
    >> of writable ptes reach 0, when the dirty bits were last cleared etc). And
    >> none of the normal asertions were triggering: eg. when any page is removed
    >> from pagecache (except truncates), it has always had all its buffers
    >> written out *after* all ptes were made readonly or unmapped. Lots of other
    >> tests and crap like that.
    >> So I tried what I should have done to start with and did an e2fsck after
    >> seeing corruption. Yes, it comes up with errors.
    > Do you recall what the errors were?

    I run e2fsck on the partition after the failure happened and here is
    what i saw, not sure if that is the same message Jan looked at:

    e2fsck 1.41.3 (12-Oct-2008)
    Warning! /dev/hda1 is mounted.
    /dev/hda1 contains a file system with errors, check forced.
    Pass 1: Checking inodes, blocks, and sizes
    Pass 2: Checking directory structure
    Pass 3: Checking directory connectivity
    Pass 4: Checking reference counts
    Pass 5: Checking group summary information
    Block bitmap differences: +74915 -195111 -224680
    Fix? no

    Free blocks count wrong for group #6 (170, counted=169).
    Fix? no

    Free blocks count wrong (10120, counted=523).
    Fix? no

    Free inodes count wrong (95678, counted=95672).
    Fix? no

    /dev/hda1: ********** WARNING: Filesystem still has errors **********

    /dev/hda1: 35938/131616 files (1.5% non-contiguous), 252936/263056 blocks


    >> Now that is unusual
    >> because that should be largely insulated from the vm: if a dirty bit gets
    >> lost, then the filesystem image should be quite happy and error-free with
    >> a hole or unwritten data there.
    >> I don't know ext? locking very well, except that it looks pretty overly
    >> complex and crufty.
    >> Usually, blocks are instantiated by write(2), under i_mutex, serialising
    >> the allocator somewhat. mmap-write blocks are instantiated at writeout
    >> time, unserialised. I moved truncate_mutex to cover the entire get_blocks
    >> function, and can no longer trigger the problem. Might be a timing issue
    >> though -- Ying, can you try this and see if you can still reproduce?
    >> I close my eyes and pick something out of a hat. a686cd89. Search for XXX.
    >> Nice. Whether or not this cased the problem, can someone please tell me
    >> why it got merged in that state?
    >> I'm leaving ext3 running for now. It looks like a nasty task to bisect
    >> ext2 down to that commit :( and I would be more interested in trying to
    >> reproduce Jan's ext3 problem, however, because I'm not too interested in
    >> diving into ext2 locking to work out exactly what is racing and how to
    >> fix it properly. I suspect it would be most productive to wire up some
    >> ioctls right into the block allocator/lookup and code up a userspace
    >> tester for it that could probably stress it a lot harder than kernel
    >> writeout can.

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