linux-kernel - Re: [PATCH 02/11] filemap: Remove filemap_check_and_keep

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Message-ID: <Y7weinAVLt0uPRa8@casper.infradead.org>
Date:   Mon, 9 Jan 2023 14:02:50 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Jeff Layton <jlayton@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 02/11] filemap: Remove filemap_check_and_keep_errors()

On Mon, Jan 09, 2023 at 08:48:49AM -0500, Jeff Layton wrote:
> On Mon, 2023-01-09 at 05:18 +0000, Matthew Wilcox (Oracle) wrote:
> > Convert both callers to use the "new" errseq infrastructure.
> 
> I looked at making this sort of change across the board alongside the
> original wb_err patches, but I backed off at the time.
> 
> With the above patch, this function will no longer report a writeback
> error that occurs before the sample. Given that writeback can happen at
> any time, that seemed like it might be an undesirable change, and I
> didn't follow through.
> 
> It is true that the existing flag-based code may miss errors too, if
> multiple tasks are test_and_clear'ing the bits, but I think the above is
> even more likely to happen, esp. under memory pressure.
> 
> To do this right, we probably need to look at these callers and have
> them track a long-term errseq_t "since" value before they ever dirty the
> pages, and then continually check-and-advance vs. that.
> 
> For instance, the main caller of the above function is jbd2. Would it be
> reasonable to add in a new errseq_t value to the jnode for tracking
> errors?

Doesn't b4678df184b3 address this problem?  If nobody has seen the
error, we return 0 instead of the current value of wb_err, ensuring
that somebody always sees the error.