linux-kernel - Re: [PATCH 0/5] Take 3 at async RPCs and no longer looping forever on signals

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Date:   Tue, 14 Feb 2023 10:34:36 +0100
From:   Christian Schoenebeck <linux_oss@...debyte.com>
To:     Dominique Martinet <asmadeus@...ewreck.org>
Cc:     v9fs-developer@...ts.sourceforge.net,
        Eric Van Hensbergen <ericvh@...il.com>,
        Latchesar Ionkov <lucho@...kov.net>,
        linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Pengfei Xu <pengfei.xu@...el.com>
Subject: Re: [PATCH 0/5] Take 3 at async RPCs and no longer looping forever on signals

On Monday, February 13, 2023 11:37:50 PM CET Dominique Martinet wrote:
> Christian Schoenebeck wrote on Mon, Feb 13, 2023 at 07:45:34PM +0100:
> > On Monday, February 13, 2023 7:26:54 PM CET Christian Schoenebeck wrote:
> > > On Saturday, February 11, 2023 8:50:18 AM CET Dominique Martinet wrote:
> > > > I've been working on async RPCs for a while and never had time to debug
> > > > the last issues this had, but by limiting the async clunks to failures
> > > > the impact is drastically smaller and I've not been able to reproduce
> > > > any more bug so far.
> > > > 
> > > > This will require some more testing and I'm tempted to say this is not
> > > > worth rushing this into the merge window next week-ish; the new problem
> > > > Jens reported with task_work isn't really new and I'd rather get this
> > > > right than rush new bugs in given the sour experience I've had with this
> > > > patch series... Hopefully it'll get in this time.
> > > > With that in mind I plan to take the patches in to my -next branch after
> > > > the merge window, so this has time to get reviewed first.
> > > > 
> > > > I'd like to measure impact on performance as well, but really spent way
> > > > more time on this than I already have, so that'll have to wait a bit.
> > > 
> > > I have not observed performance degradation, nor any (new) errors. So for
> > > this entire series:
> > > 
> > > Tested-by: Christian Schoenebeck <linux_oss@...debyte.com>
> > 
> > Sorry, strike that. :-/
> 
> Ugh! thanks for the test, I'm not making much sense of the trace (folio
> wait bit means some background write isn't happening when flushing
> before the setattr?) but I'll try to reproduce and have a look
> today/tomorrow

Yes, apparently it tries to write dirty pages of the mapped file and keeps
hanging there [fs/9p/vfs_inode_dotl.c:586]:

int v9fs_vfs_setattr_dotl(struct user_namespace *mnt_userns,
			  struct dentry *dentry, struct iattr *iattr)
{
	...

	/* Write all dirty data */
	if (S_ISREG(inode->i_mode))
		filemap_write_and_wait(inode->i_mapping); <-- Hang

	retval = p9_client_setattr(fid, &p9attr);

	...
}