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

From: Dmitriy Monakhov
Date: Wed Jun 13 2007 - 06:39:55 EST

On 14:19 ÐÑÑ 29 ÐÐÐ , akpm@xxxxxxxxxxxxxxxxxxxx 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 to find
> out what to do about this
> ------------------------------------------------------
> Subject: fs: introduce write_begin, write_end, and perform_write aops
> From: Nick Piggin <npiggin@xxxxxxx>
> These are intended to replace prepare_write and 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@xxxxxxxxxx: API design contributions, code review and fixes]
> Signed-off-by: Nick Piggin <npiggin@xxxxxxx>
> Signed-off-by: Mark Fasheh <mark.fasheh@xxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
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.

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

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) {
Fixed my "new aop block_write_begin fix" patch

3) __page_symlink:
Nick's patch add folowing code:
+ err = pagecache_write_begin(NULL, mapping, 0,PAGE_CACHE_SIZE,
symlink now consume whole page. I have only one question "WHY???".
I don't see any advantages, but where are huge list of
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

I dont prepare patch for this because i dont understand issue
witch Nick aimed to fix.

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"

5) ext3_write_end:
Before write_begin/write_end patch set we have folowing locking
But now order is oposite:
Can we got any race condition now? I'm not sure is it actual problem,
may be somebody cant describe this.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at