On 01/12/15 10:30, Brian Foster wrote:
> On Fri, Jan 09, 2015 at 03:02:53PM -0500, Michael L. Semon wrote:
>> Hi! I like this patch and am confident with it on x86. However,
>> a) it has no x86_64 coverage; and b) xfstests xfs/306 in particular
>> emits more of this output:
>>
>> buffer_io_error: nnnn callbacks suppressed
>>
>> Might someone evaluate this patch or the intent of the patch?
>>
>> The intent:
>>
>> For XFS filesystems that don't change much, such as the /boot and
>> alternate / partitions here, mount times were about 17s instead of
>> 0.4s while the log is in a wrapped state, write caches off. This
>> patch fixes the issue on v4- and v5-superblock XFS filesystems.
>>
>> xfs_repair can solve this issue short-term and also cut wrapped-log
>> mount time in half short-term for v5 file systems. Don't know if
>> that's a mkfs.xfs issue or just coincidence.
>>
>> A bisect still needs to be done to determine when the slow mount
>> behavior started. It could very well be that somebody fixed the
>> buffer_io_error messages that I saw long ago, and the solution made
>> some mounts here rather miserable.
>>
>> Thanks!
>>
>> Michael
>>
>> The patch:
>>
>> xlog_write_log_records() has an algorithm to "Greedily allocate a
>> buffer big enough...," starting with ffs(blocks), adding two sensible
>> checks, and then feeding it to a loop with checks of its own.
>>
>> However, when blocks is an odd number, the number that becomes nbblks
>> to the xlog_bwrite() function ends up being 2 (1 << 1). The most
>> obvious effect is that when the log wraps, a write of two odd-sized
>> log regions on an 8-GB XFS filesystem will take around 2049 calls
>> to xlog_bwrite() instead of the "two separate I/Os" suggested in
>> xlog_clear_stale_blocks().
>>
>> Fix this by changing the ffs(blocks) to fls(blocks).
>>
>> There is a similar ffs(blocks) check in xlog_find_verify_cycle().
>> This was not investigated.
>>
>> Signed-off-by: Michael L. Semon <mlsemon35@xxxxxxxxx>
>> ---
>> fs/xfs/xfs_log_recover.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/xfs/xfs_log_recover.c b/fs/xfs/xfs_log_recover.c
>> index a5a945f..13381eb 100644
>> --- a/fs/xfs/xfs_log_recover.c
>> +++ b/fs/xfs/xfs_log_recover.c
>> @@ -1242,7 +1242,7 @@ xlog_write_log_records(
>> * a smaller size. We need to be able to write at least a
>> * log sector, or we're out of luck.
>> */
>> - bufblks = 1 << ffs(blocks);
>> + bufblks = 1 << fls(blocks);
>
> Interesting, it does seem like there is a bug here. Thanks for the
> test code to help reproduce.
>
> The fix seems reasonable to me, but I'm also wondering if there is at
> least one other bug in this code. In the middle of that loop, we have
> the following:
>
> ...
>
> /* We may need to do a read at the end to fill in part of
> * the buffer in the final sector not covered by the write.
> * If this is the same sector as the above read, skip it.
> */
> ealign = round_down(end_block, sectbb);
> if (j == 0 && (start_block + endcount > ealign)) {
> offset = bp->b_addr + BBTOB(ealign - start_block);
> error = xlog_bread_offset(log, ealign, sectbb,
> bp, offset);
> ...
> }
> ...
>
> ... but how have we really confirmed whether the end sector is
> equivalent to the first sector? It looks to me that we operate at basic
> block granularity but log I/O is managed at log sector alignment. So if
> the start basic block is not sector aligned, we read in the first sector
> and add records at the associated buffer offset. Similar if the end
> block is not sector aligned. If the buffer size spans multiple sectors
> and the start and end are not aligned, it looks like we could skip the
> read of the final sector.
> Perhaps I'm missing some context as to why this wouldn't occur..? It
> also seems strange that the offset calculation above uses start_block as
> the baseline start block value of the buffer, but the pre-loop balign
> code suggests the buffer might not be aligned to start_block...
>
> Brian
I'm currently stumped on getting this code to fire. For that matter, all
blk_no and nbblks numbers are coming in to xlog_bwrite() neatly pre-
rounded, so the rounding functions in there don't change anything. In
all, an unsuccessful testing effort on my part.
Maybe the "env MKFS_OPTIONS='-m crc=1,finobt=1 -s size=4096' ./check -g
auto" xfstests run will cough something up. It will be waiting at home,
at the end of the day.
Should I use fdisk on a spare disk and deliberately misalign the
partitions? Otherwise, there's a struggle to find misalignments, and
my idea bucket was not very full, anyway ;-)
Thanks!
Michael
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