Re: [PATCH 1/1] fs/mpage.c: forgotten WRITE_SYNC in case of data integrity write

Roman Peniaev
2014-03-12T14:29:09Z

Jens,

could you please explain the real purpose of WAIT_SYNC?
In case of wbc->sync_mode == WB_SYNC_ALL.
Because my current understanding is if writeback control has
WB_SYNC_ALL everything
should be submitted with WAIT_SYNC.

--
Roman

On Wed, Feb 19, 2014 at 10:38 AM, Roman Peniaev <r.peniaev@gmail.com> wrote:
> (my previous email was rejected by vger.kernel.org because google web
> sent it as html.
> will resend the same one in plain text mode)
>
>> What do REQ_SYNC and REQ_NOIDLE actually *do*?
>
> Yep, this REQ_SYNC is very confusing to me.
> First of all according to the sources of old school block buffer filesystems
> (e.g. ext2) we can get this stack in case of fsync call:
>
> __filemap_fdatawrite_range(mapping, 0, LLONG_MAX, sync_mode)
> do_writepages(mapping, &wbc)
> mapping->a_ops->writepages(page, wbc)
> (ext2_writepages)
> mpage_writepages(mapping, wbc, fat_get_block);
> write_cache_pages(mapping, wbc, __mpage_writepage, &mpd)
> __mpage_writepage(page, wbc, data)
>>>>>> mpage_bio_submit(WRITE, bio) >>>> why WRITE? not WRITE_SYNC in case of WB_SYNC_ALL?
> <or in case of not contiguous buffers>
> mapping->a_ops->writepage(page, wbc)
> (ext2_writepage)
> block_write_full_page(page, fat_get_block, wbc)
> block_write_full_page_endio(page, get_block, wbc,
> end_buffer_async_write)
> __block_write_full_page(inode, page, get_block, wbc,
> handler);
> submit_bh(WRITE_SYNC)
>
> So, it turns out to be that some bios for the same dirty range
> can be submitted with REQ_WRITE|REQ_SYNC|REQ_NOIDLE and some of
> the bios only with REQ_WRITE.
> (according to the comment of __mpage_writepage:
> * If all blocks are found to be contiguous then the page can go into the
> * BIO. Otherwise fall back to the mapping's writepage().
> )
>
> Also, it seems to me that all over the kernel WRITE_SYNC has meaning of:
> 1. try to get the block on-disk faster
> 2. if I have to do flush - mark my bio with WRITE_SYNC and wait for result
>
> My patch is an attempt to make some unification in case of fsync call.
>
> --
> Roman
>
>
> On Wed, Feb 19, 2014 at 8:59 AM, Andrew Morton
> <akpm@linux-foundation.org> wrote:
>> On Sun, 16 Feb 2014 11:54:28 +0900 Roman Pen <r.peniaev@gmail.com> wrote:
>>
>>> In case of wbc->sync_mode == WB_SYNC_ALL we need to do data integrity write,
>>> thus mark request as WRITE_SYNC.
>>
>> gargh, the documentation for this stuff is useless.
>>
>> What do REQ_SYNC and REQ_NOIDLE actually *do*?
>>
>> For mpage writes, REQ_NOIDLE appears to be incorrect - we very much
>> expect that there will be more writes and that they will be contiguous
>> with this one. But we won't be waiting on this write before submitting
>> more writes, so perhaps REQ_NOIDLE is at least harmless.
>>
>> I dunno about REQ_SYNC - it requires delving into the bowels of CFQ
>> and we shouldn't need to do that.
>>
>> Jens. Help. How is a poor kernel reader supposed to work this out?
>>
>>> --- a/fs/mpage.c
>>> +++ b/fs/mpage.c
>>> @@ -462,6 +462,7 @@ static int __mpage_writepage(struct page *page, struct writeback_control *wbc,
>>> struct buffer_head map_bh;
>>> loff_t i_size = i_size_read(inode);
>>> int ret = 0;
>>> + int wr = (wbc->sync_mode == WB_SYNC_ALL ? WRITE_SYNC : WRITE);
>>>
>>> if (page_has_buffers(page)) {
>>> struct buffer_head *head = page_buffers(page);
>>> @@ -570,7 +571,7 @@ page_is_mapped:
>>> * This page will go to BIO. Do we need to send this BIO off first?
>>> */
>>> if (bio && mpd->last_block_in_bio != blocks[0] - 1)
>>> - bio = mpage_bio_submit(WRITE, bio);
>>> + bio = mpage_bio_submit(wr, bio);
>>>
>>> alloc_new:
>>> if (bio == NULL) {
>>> @@ -587,7 +588,7 @@ alloc_new:
>>> */
>>> length = first_unmapped << blkbits;
>>> if (bio_add_page(bio, page, length, 0) < length) {
>>> - bio = mpage_bio_submit(WRITE, bio);
>>> + bio = mpage_bio_submit(wr, bio);
>>> goto alloc_new;
>>> }
>>>
>>> @@ -620,7 +621,7 @@ alloc_new:
>>> set_page_writeback(page);
>>> unlock_page(page);
>>> if (boundary || (first_unmapped != blocks_per_page)) {
>>> - bio = mpage_bio_submit(WRITE, bio);
>>> + bio = mpage_bio_submit(wr, bio);
>>> if (boundary_block) {
>>> write_boundary_block(boundary_bdev,
>>> boundary_block, 1 << blkbits);
>>> @@ -632,7 +633,7 @@ alloc_new:
>>>
>>> confused:
>>> if (bio)
>>> - bio = mpage_bio_submit(WRITE, bio);
>>> + bio = mpage_bio_submit(wr, bio);
>>>
>>> if (mpd->use_writepage) {
>>> ret = mapping->a_ops->writepage(page, wbc);
>>> @@ -688,8 +689,11 @@ mpage_writepages(struct address_space *mapping,
>>> };
>>>
>>> ret = write_cache_pages(mapping, wbc, __mpage_writepage, &mpd);
>>> - if (mpd.bio)
>>> - mpage_bio_submit(WRITE, mpd.bio);
>>> + if (mpd.bio) {
>>> + int wr = (wbc->sync_mode == WB_SYNC_ALL ?
>>> + WRITE_SYNC : WRITE);
>>> + mpage_bio_submit(wr, mpd.bio);
>>> + }
>>> }
>>> blk_finish_plug(&plug);
>>> return ret;
>>> @@ -706,8 +710,11 @@ int mpage_writepage(struct page *page, get_block_t get_block,
>>> .use_writepage = 0,
>>> };
>>> int ret = __mpage_writepage(page, wbc, &mpd);
>>> - if (mpd.bio)
>>> - mpage_bio_submit(WRITE, mpd.bio);
>>> + if (mpd.bio) {
>>> + int wr = (wbc->sync_mode == WB_SYNC_ALL ?
>>> + WRITE_SYNC : WRITE);
>>> + mpage_bio_submit(wr, mpd.bio);
>>> + }
>>> return ret;
>>> }
>>> EXPORT_SYMBOL(mpage_writepage);
>>