On Thu, Mar 24, 2011 at 10:43 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > Michel, > > can you take a look at this bug report? It looks like a regression > in your mlock handling changes. I had a quick look and at this point I can describe how the patch will affect behavior of this test, but not why this causes a deadlock with xfs. The test creates a writable, shared mapping of a file that does not have data blocks allocated on disk, and also uses the MAP_POPULATE flag. Before 5ecfda041e4b4bd858d25bbf5a16c2a6c06d7272, make_pages_present during the mmap would cause data blocks to get allocated on disk with an xfs_vm_page_mkwrite call, and then the file pages would get mapped as writable ptes. After 5ecfda041e4b4bd858d25bbf5a16c2a6c06d7272, make_pages_present does NOT cause data blocks to get allocated on disk. Instead, xfs_vm_readpages is called, which (I suppose) does not allocate the data blocks and returns zero filled pages instead, which get mapped as readonly ptes. Later, the test tries writing into the mmap'ed block, causing minor page faults, xfs_vm_page_mkwrite calls and data block allocations to occur. Regarding the deadlock: I am curious to see if it could be made to happen before 5ecfda041e4b4bd858d25bbf5a16c2a6c06d7272. Could you test what happens if you remove the MAP_POPULATE flag from your mmap call, and instead read all pages from userspace right after the mmap ? I expect you would then be able to trigger the deadlock before 5ecfda041e4b4bd858d25bbf5a16c2a6c06d7272. This leaves the issue of the change of behavior for MAP_POPULATE on ftruncated file holes. I'm not sure what to say there though, because MAP_POPULATE is documented to cause file read-ahead (and it still does after 5ecfda041e4b4bd858d25bbf5a16c2a6c06d7272), but that doesn't say anything about block allocation. Hope this helps, -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies. _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
