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Date:   Mon, 9 Mar 2020 12:22:40 -0700
From:   Andres Freund <andres@anarazel.de>
To:     Jeff Layton <jlayton@redhat.com>
Cc:     David Howells <dhowells@redhat.com>, torvalds@linux-foundation.org,
        viro@zeniv.linux.org.uk, Theodore Ts'o <tytso@mit.edu>,
        Stefan Metzmacher <metze@samba.org>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        linux-ext4@vger.kernel.org, Aleksa Sarai <cyphar@cyphar.com>,
        Trond Myklebust <trond.myklebust@hammerspace.com>,
        Anna Schumaker <anna.schumaker@netapp.com>,
        linux-nfs@vger.kernel.org, linux-api@vger.kernel.org,
        raven@themaw.net, mszeredi@redhat.com, christian@brauner.io,
        jannh@google.com, darrick.wong@oracle.com, kzak@redhat.com,
        linux-fsdevel@vger.kernel.org,
        linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 00/14] VFS: Filesystem information [ver #18]
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Hi,

On 2020-03-09 13:50:59 -0400, Jeff Layton wrote:
> The PostgreSQL devs asked a while back for some way to tell whether
> there have been any writeback errors on a superblock w/o having to do
> any sort of flush -- just "have there been any so far".

Indeed.


> I sent a patch a few weeks ago to make syncfs() return errors when there
> have been writeback errors on the superblock. It's not merged yet, but
> once we have something like that in place, we could expose info from the
> errseq_t to userland using this interface.

I'm still a bit worried about the details of errseq_t being exposed to
userland. Partially because it seems to restrict further evolution of
errseq_t, and partially because it will likely up with userland trying
to understand it (it's e.g. just too attractive to report a count of
errors etc).

Is there a reason to not instead report a 64bit counter instead of the
cookie? In contrast to the struct file case we'd only have the space
overhead once per superblock, rather than once per #files * #fd. And it
seems that the maintenance of that counter could be done without
widespread changes, e.g. instead/in addition to your change:

> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index ccb14b6a16b5..897439475315 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -51,7 +51,10 @@ static inline void mapping_set_error(struct address_space *mapping, int error)
>  		return;
>
>  	/* Record in wb_err for checkers using errseq_t based tracking */
> -	filemap_set_wb_err(mapping, error);
> +	__filemap_set_wb_err(mapping, error);
> +
> +	/* Record it in superblock */
> +	errseq_set(&mapping->host->i_sb->s_wb_err, error);
>
>  	/* Record it in flags for now, for legacy callers */
>  	if (error == -ENOSPC)

Btw, seems like mapping_set_error() should have a non-inline cold path?

Greetings,

Andres Freund