Messages in this thread | | | Date | Fri, 06 Aug 2004 12:19:23 +1000 | From | Nick Piggin <> | Subject | Re: [PATCH] VFS readahead bug in 2.6.8-rc[1-3] |
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Nick Piggin wrote:
> Phillip Lougher wrote: > >> Hi, >> >> There is a readahead bug in do_generic_mapping_read (filemap.c). This >> bug appears to have been introduced in 2.6.8-rc1. Specifically the bug >> is caused by an incorrect code change which causes VFS to call >> readpage() for indexes beyond the end of files where the file length is >> zero or a 4k multiple. >> >> In Squashfs this causes a variety of almost immediate OOPes because >> Squashfs trusts the VFS not to pass invalid index values. For other >> filesystems it may also be causing subtle bugs. I have received >> prune_dcache oopes similar to Gene Heskett's (which was also >> pointer corruption), and so it may fix this and other reported >> readahead bugs. >> >> The patch is against 2.6.8-rc3. >> > > Good work - bug is mine, sorry. >
On second thought, maybe not. I think your filesystem is at fault.
Firstly, there possibly is a bug in do_pagecache_readahead which allows it to read off the end of the file in a completely serialised situation. But even so, notice the absence of locking - i_size can change at any time can't it? Then your fix will blow up when you race with a truncate, right?
I think you need to handle reading off the end of the file properly: I'm not too familiar with this code, but IIRC you are allowed to setup pagecache past the end of the file - it gets handled correctly in do_generic_mapping_read, and ends up falling off the LRU. This should help you.
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