From: Jan Kara <jack@suse.cz>
To: Ritesh Harjani <ritesh.list@gmail.com>
Cc: Jan Kara <jack@suse.cz>, Baokun Li <libaokun1@huawei.com>,
linux-ext4@vger.kernel.org, tytso@mit.edu,
adilger.kernel@dilger.ca, linux-kernel@vger.kernel.org,
yi.zhang@huawei.com, yebin10@huawei.com, yukuai3@huawei.com
Subject: Re: [PATCH 2/2] ext4: correct the judgment of BUG in ext4_mb_normalize_request
Date: Tue, 24 May 2022 11:39:33 +0200 [thread overview]
Message-ID: <20220524093933.bittzsrrpddttnab@quack3.lan> (raw)
In-Reply-To: <20220524062655.ddiltnfxxhlelfgb@riteshh-domain>
On Tue 24-05-22 11:56:55, Ritesh Harjani wrote:
> On 22/05/23 11:08PM, Jan Kara wrote:
> > On Tue 24-05-22 01:38:44, Ritesh Harjani wrote:
> > > On 22/05/21 09:42PM, Baokun Li wrote:
> > > > When either of the "start + size <= ac->ac_o_ex.fe_logical" or
> > > > "start > ac->ac_o_ex.fe_logical" conditions is met, it indicates
> > > > that the fe_logical is not in the allocated range.
> > >
> > > Sounds about right to me based on the logic in ext4_mb_use_inode_pa().
> > > We try to allocate/preallocate such that ac->ac_o_ex.fe_logical should fall
> > > within the preallocated range. So if our start or start + size doesn't include
> > > fe_logical then it is a bug in the ext4_mb_normalize_request() logic.
> >
> > I agree ac->ac_o_ex.fe_logical is a goal block. But AFAIK it never was a
> > hard guarantee that we would allocate extent that includes that block. It
>
> Agree that the guarantee is not about the extent which finally gets allocated.
> It is only within ext4_mb_normalize_request() that the "start" and "size"
> variable calculations is done in such a way that the ac->ac_o_ex.fe_logical
> block will always fall within the "start" & "end" boundaries after
> normalization.
>
> That is how it also updates the goal block at the end too. ac->ac_g_ex.
>
> > was always treated as a hint only. In particular if you look at the logic
> > in ext4_mb_normalize_request() it does shift the start of the allocation to
> > avoid preallocated ranges etc.
>
> Yes, I checked the logic of ext4_mb_normalize_request() again.
> As I see it (I can be wrong, so please correct me), there is always an attempt
> to make "start" & "start + size" such that it covers ac->ac_o_ex.fe_logical
> except just one change where we are trimming the size of the request to only
> EXT4_BLOCKS_PER_GROUP.
>
> For e.g. when it compares against preallocated ranges. It has a BUG() which
> checks if the ac->ac_o_ex.fe_logical already lies in the preallocated range.
> Because then we should never have come here to do allocation of a new block.
>
> 4143 /* PA must not overlap original request */
> 4144 BUG_ON(!(ac->ac_o_ex.fe_logical >= pa_end ||
> 4145 ac->ac_o_ex.fe_logical < pa->pa_lstart));
> <...>
> 4152 BUG_ON(pa->pa_lstart <= start && pa_end >= end);
>
> Then after skipping the preallocated regions which doesn't fall in between
> "start" and "end"...
>
> 4147 /* skip PAs this normalized request doesn't overlap with */
> 4148 if (pa->pa_lstart >= end || pa_end <= start) {
> 4149 spin_unlock(&pa->pa_lock);
> 4150 continue;
> 4151 }
>
> ...it adjusts "start" and "end" boundary according to allocated PAs boundaries
> such that fe_logical is always in between "start" and "end".
>
> 4154 /* adjust start or end to be adjacent to this pa */
> 4155 if (pa_end <= ac->ac_o_ex.fe_logical) {
> 4156 BUG_ON(pa_end < start);
> 4157 start = pa_end;
> 4158 } else if (pa->pa_lstart > ac->ac_o_ex.fe_logical) {
> 4159 BUG_ON(pa->pa_lstart > end);
> 4160 end = pa->pa_lstart;
> 4161 }
>
>
>
> > so I don't see how we are guaranteed that
> > ext4_mb_normalize_request() will result in an allocation request that
> > includes ac->ac_o_ex.fe_logical.
>
> It could be I am wrong, but looks like ext4_mb_normalize_request() keeps
> ac->ac_o_ex.fe_logical within "start" and "end" of allocation request.
> And then updates the goal block.
>
> 4196 ac->ac_g_ex.fe_logical = start;
> 4197 ac->ac_g_ex.fe_len = EXT4_NUM_B2C(sbi, size);
>
> Thoughts?
Right, after some more inspection the only thing I'm concerned about is:
/* don't cover already allocated blocks in selected range */
if (ar->pleft && start <= ar->lleft) {
size -= ar->lleft + 1 - start;
start = ar->lleft + 1;
}
which can shift start beyond ac->ac_o_ex.fe_logical if the block would be
already allocated. But I guess in that case we should not be calling
ext4_mb_normalize_request()? ... some more code digging .. Yes, that is
guaranteed in how lleft is initialized in ext4_ext_map_blocks(). So OK, I
withdraw my objection to the stronger check but the changelog really needs
to do a better job to explain why the stronger condition should be true.
Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR
next prev parent reply other threads:[~2022-05-24 9:39 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-21 13:42 [PATCH 0/2] ext4: fix two bugs in ext4_mb_normalize_request Baokun Li
2022-05-21 13:42 ` [PATCH 1/2] ext4: fix bug_on ext4_mb_use_inode_pa Baokun Li
2022-05-23 9:29 ` Jan Kara
2022-05-23 9:58 ` Lukas Czerner
[not found] ` <2525e39a-5be9-bae1-b77d-60f583892868@huawei.com>
2022-05-24 12:11 ` Lukas Czerner
2022-05-24 12:42 ` Baokun Li
2022-05-23 19:51 ` Ritesh Harjani
2022-05-21 13:42 ` [PATCH 2/2] ext4: correct the judgment of BUG in ext4_mb_normalize_request Baokun Li
2022-05-23 9:40 ` Jan Kara
[not found] ` <3755e40b-f817-83df-b239-b0697976c272@huawei.com>
2022-05-24 9:30 ` Jan Kara
2022-05-24 13:44 ` Baokun Li
2022-05-25 11:29 ` Jan Kara
2022-05-26 1:16 ` Baokun Li
2022-05-23 10:05 ` Lukas Czerner
2022-05-23 20:08 ` Ritesh Harjani
2022-05-23 21:08 ` Jan Kara
2022-05-24 6:26 ` Ritesh Harjani
2022-05-24 9:39 ` Jan Kara [this message]
2022-05-24 17:31 ` Ritesh Harjani
2022-05-25 12:12 ` Jan Kara
2022-05-24 6:09 ` Baokun Li
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20220524093933.bittzsrrpddttnab@quack3.lan \
--to=jack@suse.cz \
--cc=adilger.kernel@dilger.ca \
--cc=libaokun1@huawei.com \
--cc=linux-ext4@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=ritesh.list@gmail.com \
--cc=tytso@mit.edu \
--cc=yebin10@huawei.com \
--cc=yi.zhang@huawei.com \
--cc=yukuai3@huawei.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).