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Commit 81b0c871 authored by Andrew Morton's avatar Andrew Morton Committed by Linus Torvalds
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[PATCH] generic_file_buffered_write(): handle zero-length iovec segments 



The recent generic_file_write() deadlock fix caused
generic_file_buffered_write() to loop inifinitely when presented with a
zero-length iovec segment.  Fix.

Note that this fix deliberately avoids calling ->prepare_write(),
->commit_write() etc with a zero-length write.  This is because I don't trust
all filesystems to get that right.

This is a cautious approach, for 2.6.17.x.  For 2.6.18 we should just go ahead
and call ->prepare_write() and ->commit_write() with the zero length and fix
any broken filesystems.  So I'll make that change once this code is stabilised
and backported into 2.6.17.x.

The reason for preferring to call ->prepare_write() and ->commit_write() with
the zero-length segment: a zero-length segment _should_ be sufficiently
uncommon that this is the correct way of handling it.  We don't want to
optimise for poorly-written userspace at the expense of well-written
userspace.

Cc: "Vladimir V. Saveliev" <vs@namesys.com>
Cc: Neil Brown <neilb@suse.de>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Chris Wright <chrisw@sous-sol.org>
Cc: Greg KH <greg@kroah.com>
Cc: <stable@kernel.org>
Cc: walt <wa1ter@myrealbox.com>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 0686cd8f

mm/filemap.c

+8 −1
Original line number Original line Diff line number Diff line
@@ -2125,6 +2125,12 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov, 
			break;

			break;

		}

		}





		if (unlikely(bytes == 0)) {

			status = 0;

			copied = 0;

			goto zero_length_segment;

		}



		status = a_ops->prepare_write(file, page, offset, offset+bytes);

		status = a_ops->prepare_write(file, page, offset, offset+bytes);

		if (unlikely(status)) {

		if (unlikely(status)) {

			loff_t isize = i_size_read(inode);

			loff_t isize = i_size_read(inode);
@@ -2154,7 +2160,8 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov, 
			page_cache_release(page);

			page_cache_release(page);

			continue;

			continue;

		}

		}

		if (likely(copied > 0)) {

zero_length_segment:

		if (likely(copied >= 0)) {

			if (!status)

			if (!status)

				status = copied;

				status = copied;

mm/filemap.h

+2 −2
Original line number Original line Diff line number Diff line
@@ -88,7 +88,7 @@ filemap_set_next_iovec(const struct iovec **iovp, size_t *basep, size_t bytes) 
	const struct iovec *iov = *iovp;

	const struct iovec *iov = *iovp;

	size_t base = *basep;

	size_t base = *basep;





	while (bytes) {

	do {

		int copy = min(bytes, iov->iov_len - base);

		int copy = min(bytes, iov->iov_len - base);





		bytes -= copy;

		bytes -= copy;
@@ -97,7 +97,7 @@ filemap_set_next_iovec(const struct iovec **iovp, size_t *basep, size_t bytes) 
			iov++;

			iov++;

			base = 0;

			base = 0;

		}

		}

	}

	} while (bytes);

	*iovp = iov;

	*iovp = iov;

	*basep = base;

	*basep = base;

}

}