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Re: [PATCH 04/19] Make effect of PF_FSTRANS to disable __GFP_FS universa

To: NeilBrown <neilb@xxxxxxx>
Subject: Re: [PATCH 04/19] Make effect of PF_FSTRANS to disable __GFP_FS universal.
From: NeilBrown <neilb@xxxxxxx>
Date: Thu, 17 Apr 2014 11:03:50 +1000
Cc: Dave Chinner <david@xxxxxxxxxxxxx>, linux-mm@xxxxxxxxx, linux-nfs@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, Ming Lei <ming.lei@xxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140416161726.51b506e2@xxxxxxxxxxxxxx>
References: <20140416033623.10604.69237.stgit@xxxxxxxxxxxxxx> <20140416040336.10604.58240.stgit@xxxxxxxxxxxxxx> <20140416053756.GC15995@dastard> <20140416161726.51b506e2@xxxxxxxxxxxxxx>
On Wed, 16 Apr 2014 16:17:26 +1000 NeilBrown <neilb@xxxxxxx> wrote:

> On Wed, 16 Apr 2014 15:37:56 +1000 Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> 
> > On Wed, Apr 16, 2014 at 02:03:36PM +1000, NeilBrown wrote:

> > > - /*
> > > -  * Given that we do not allow direct reclaim to call us, we should
> > > -  * never be called while in a filesystem transaction.
> > > -  */
> > > - if (WARN_ON(current->flags & PF_FSTRANS))
> > > -         goto redirty;
> > 
> > We still need to ensure this rule isn't broken. If it is, the
> > filesystem will silently deadlock in delayed allocation rather than
> > gracefully handle the problem with a warning....
> 
> Hmm... that might be tricky.  The 'new' PF_FSTRANS can definitely be set when
> xfs_vm_writepage is called and we really want the write to happen.
> I don't suppose there is any other way to detect if a transaction is
> happening?

I've been thinking about this some more....

That code is in xfs_vm_writepage which is only called as ->writepage.
xfs never calls that directly so it could only possibly be called during
reclaim?

We know that doesn't happen, but if it does then PF_MEMALLOC would be set,
but PF_KSWAPD would not... and you already have a test for that.

How about every time we set PF_FSTRANS, we store the corresponding
xfs_trans_t in current->journal_info, and clear that field when PF_FSTRANS is
cleared.  Then xfs_vm_writepage can test for current->journal_info being
clear.
That is the field that several other filesystems use to keep track of the
'current' transaction.
??

I don't know what xfs_trans_t we would use in xfs_bmapi_allocate_worker, but
I suspect you do :-)

Thanks,
NeilBrown

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