Messages in this thread | | | Date | Thu, 29 Aug 2013 17:01:06 +0400 | From | Maxim Patlasov <> | Subject | Re: [PATCH] fuse: hotfix truncate_pagecache() issue |
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Hi,
08/29/2013 01:25 PM, Miklos Szeredi пишет: > On Wed, Aug 28, 2013 at 04:21:46PM +0400, Maxim Patlasov wrote: >> The way how fuse calls truncate_pagecache() from fuse_change_attributes() >> is completely wrong. Because, w/o i_mutex held, we never sure whether >> 'oldsize' and 'attr->size' are valid by the time of execution of >> truncate_pagecache(inode, oldsize, attr->size). In fact, as soon as we >> released fc->lock in the middle of fuse_change_attributes(), we completely >> loose control of actions which may happen with given inode until we reach >> truncate_pagecache. The list of potentially dangerous actions includes mmap-ed >> reads and writes, ftruncate(2) and write(2) extending file size. >> >> The typical outcome of doing truncate_pagecache() with outdated arguments is >> data corruption from user point of view. This is (in some sense) acceptable >> in cases when the issue is triggered by a change of the file on the server >> (i.e. externally wrt fuse operation), but it is absolutely intolerable in >> scenarios when a single fuse client modifies a file without any external >> intervention. A real life case I discovered by fsx-linux looked like this: >> >> 1. Shrinking ftruncate(2) comes to fuse_do_setattr(). The latter sends >> FUSE_SETATTR to the server synchronously, but before getting fc->lock ... >> 2. fuse_dentry_revalidate() is asynchronously called. It sends FUSE_LOOKUP >> to the server synchronously, then calls fuse_change_attributes(). The latter >> updates i_size, releases fc->lock, but before comparing oldsize vs attr->size.. >> 3. fuse_do_setattr() from the first step proceeds by acquiring fc->lock and >> updating attributes and i_size, but now oldsize is equal to outarg.attr.size >> because i_size has just been updated (step 2). Hence, fuse_do_setattr() >> returns w/o calling truncate_pagecache(). >> 4. As soon as ftruncate(2) completes, the user extends file size by write(2) >> making a hole in the middle of file, then reads data from the hole either by >> read(2) or mmap-ed read. The user expects to get zero data from the hole, but >> gets stale data because truncate_pagecache() is not executed yet. >> >> The patch is a hotfix resolving the issue in a simplistic way: let's skip >> dangerous i_size update and truncate_pagecache if an operation changing file >> size is in progress. This simplistic approach looks correct for the cases >> w/o external changes. And to handle them properly, more sophisticated and >> intrusive techniques (e.g. NFS-like one) would be required. I'd like >> to postpone it until the issue is well discussed on the mailing list(s). > Thanks for the analysis! > > Okay, what about this even more simplistic approach? > > Not tested, but I think it addresses the very crux of the issue: not truncating > the page cache even though we should.
The patch looks fine, but it solves only one side of the problem (exactly what you formulated above). Another side is opposite: truncating page cache too late, when the state of inode changed significantly. The beginning may be as in the scenario above: fuse_dentry_revalidate() discovered that i_size changed (due to our own fuse_do_setattr()) and is going to call truncate_pagecache() for some 'new_size' it believes valid right now. But by the time that particular truncate_pagecache() is called, a lot of things may happen:
1) fuse_do_setattr() called truncate_pagecache() according to your patch 2) the file was extended either by write(2) or ftruncate(2) or fallocate(2). 3) mmap-ed write make a page in the extended region dirty
The result will be the lost of data user wrote on the step '3)'. (my patch solves the issue at least for all cases w/o external changes)
> > AFAICS there's no such issue with write(2) or fallocate(2). But I haven't > thought about this very deeply.
I added bits to the fuse_perform_write() to address that other side of the issue. Fixing fallocate is also required, but I postponed it until you include that another fix for fallocate (incorrect use of fuse_set_nowrite()).
Thanks, Maxim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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