Re: + fs-introduce-write_begin-write_end-and-perform_write-aops.patch added to -mm tree

Dmitriy Monakhov
2007-06-13T15:51:25Z

On 13:43 Срд 13 Июн , Nick Piggin wrote:
> On Wed, Jun 13, 2007 at 05:40:05PM +0400, Dmitriy Monakhov wrote:
> > On 14:19 ?????? 29 ?????? , akpm@linux-foundation.org wrote:
> > >
> > > The patch titled
> > > fs: introduce write_begin, write_end, and perform_write aops
> > > has been added to the -mm tree. Its filename is
> > > fs-introduce-write_begin-write_end-and-perform_write-aops.patch
> > >
> > > *** Remember to use Documentation/SubmitChecklist when testing your code ***
> > >
> > > See http://www.zip.com.au/~akpm/linux/patches/stuff/added-to-mm.txt to find
> > > out what to do about this
> > >
> > > ------------------------------------------------------
> > > Subject: fs: introduce write_begin, write_end, and perform_write aops
> > > From: Nick Piggin <npiggin@suse.de>
> > >
> > > These are intended to replace prepare_writ eand commit_write with more
> > > flexible alternatives that are also able to avoid the buffered write
> > > deadlock problems efficiently (which prepare_write is unable to do).
> > >
> > > [mark.fasheh@oracle.com: API design contributions, code review and fixes]
> > > Signed-off-by: Nick Piggin <npiggin@suse.de>
> > > Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>
> > > Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> > I've finaly find time to review Nick's "write_begin/write_end aop" patch set.
> > And i have some fixes and questions. My be i've missed somthing and it was
> > already disscussed, but i cant find in LKML.
>
> Thanks, that's really helpful.
>
>
> > 1) loop dev:
> > loop.c code itself is not perfect. In fact before Nick's patch
> > partial write was't possible. Assumption what write chunks are
> > always page aligned is realy weird ( see "index++" line).
> > Fixed by "new aop loop fix" patch
>
> I think you're right, fix looks good.
>
>
> > 2)block_write_begin:
> > After we enter to block_write_begin with *pagep == NULL and
> > some page was grabed we remember this page in *pagep
> > And if __block_prepare_write() we have to clear *pagep , as
> > it was before. Because this may confuse caller.
> > for example caller may have folowing code:
> > ret = block_write_begin(..., pagep,...)
> > if (ret && *pagep != NULL) {
> > unlock_page(*pagep);
> > page_cache_release(*pagep);
> > }
> > Fixed my "new aop block_write_begin fix" patch
>
> I don't think the caller can rely on that if it returns failure.
> But that is more defensive I guess. Maybe setting it to 1 or
> so would catch abusers.
>
>
> > 3) __page_symlink:
> > Nick's patch add folowing code:
> > + err = pagecache_write_begin(NULL, mapping, 0,PAGE_CACHE_SIZE,
> > + AOP_FLAG_UNINTERRUPTIBLE, &page,&fsdata);
> > symlink now consume whole page. I have only one question "WHY???".
> > I don't see any advantages, but where are huge list of
> > dissdvantages:
> > a) fs with blksize == 1k and pagesize == 16k after this patch
> > waste more than 10x times disk space for nothing.
> > b) What happends if we want use fs with blksize == 4k on i386
> > after it was used by ia64 ??? (before this patch it was
> > possible).
One more visiable effect caused by wrong symlink size:
fsstress cause folowing error:
<LOG BEGIN>
EXT3-fs unexpected failure: !buffer_revoked(bh);
inconsistent data on disk
ext3_forget: aborting transaction: IO failure in __ext3_journal_revoke
ext3_abort called.

EXT3-fs error (device dm-4): ext3_forget: error -5 when attempting
revoke
Remounting filesystem read-only
Aborting journal on device dm-4.
journal commit I/O error
journal commit I/O error
journal commit I/O error
journal commit I/O error
journal commit I/O error
journal commit I/O error
EXT3-fs error (device dm-4) in ext3_free_blocks_sb: Journal has aborted

journal commit I/O error
journal commit I/O error
EXT3-fs error (device dm-4) in ext3_reserve_inode_write: Journal has
aborted
EXT3-fs error (device dm-4) in ext3_truncate: IO failure
journal commit I/O error
journal commit I/O error
journal commit I/O error
journal commit I/O error
EXT3-fs error (device dm-4) in ext3_reserve_inode_write: Journal has
aborted
EXT3-fs error (device dm-4) in ext3_orphan_del: Journal has aborted
EXT3-fs error (device dm-4) in ext3_reserve_inode_write: Journal has
aborted
EXT3-fs error (device dm-4) in ext3_delete_inode: IO failure
<LOG END>

After symlink size was fixed to len-1 problem dissappeared.
> >
> > I dont prepare patch for this because i dont understand issue
> > witch Nick aimed to fix.
>
> I don't know why myself :P I think it would be just fine to use
> len-1 as it did previously, so it must have been a typo?
>
>
> > 4) iov_iter_fault_in_readable:
> > Function prerform check for signgle region, with out respect to
> > segment nature of iovec, For example writev no longer works :) :
> > writev(3, [{"\1", 1}, {"\2"..., 4096}], 2) = -1 EFAULT (Bad address)
> > this hidden bug, and it was invisiable because XXXX_fault_in_readable
> > return value was ignored before. Lets iov_iter_fault_in_readable
> > perform checks for all segments.
> > Fixed by :"iov_iter_fault_in_readable fix"
>
> OK thanks. I would rather just change this to use the length of
> the first iovec always (prefaulting multiple iovecs would have to
> be benchmarked on real apps that make heavy use of writev, I
> suspect).
>
>
> > 5) ext3_write_end:
> > Before write_begin/write_end patch set we have folowing locking
> > order:
> > stop_journal(handle);
> > unlock_page(page);
> > But now order is oposite:
> > unlock_page(page);
> > stop_journal(handle);
> > Can we got any race condition now? I'm not sure is it actual problem,
> > may be somebody cant describe this.
>
> Can we just change it to the original order? That would seem to be
> safest unless one of the ext3 devs explicitly acks it.
> -
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