linux-ext4 - Re: [PATCH] mm: save current->journal_info before calling fault/page

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Date:   Wed, 13 Dec 2017 18:30:22 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Yan, Zheng" <zyan@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        ceph-devel@...r.kernel.org, linux-ext4@...r.kernel.org,
        viro@...iv.linux.org.uk, jlayton@...hat.com, linux-mm@...ck.org
Subject: Re: [PATCH] mm: save current->journal_info before calling
 fault/page_mkwrite

On Thu, 14 Dec 2017 10:20:18 +0800 "Yan, Zheng" <zyan@...hat.com> wrote:

> >> +	/*
> >> +	 * If the fault happens during write_iter() copies data from
> >> +	 * userspace, filesystem may have set current->journal_info.
> >> +	 * If the userspace memory is mapped to a file on another
> >> +	 * filesystem, fault handler of the later filesystem may want
> >> +	 * to access/modify current->journal_info.
> >> +	 */
> >> +	current->journal_info = NULL;
> >> 	ret = vma->vm_ops->fault(vmf);
> >> +	/* Restore original journal_info */
> >> +	current->journal_info = old_journal_info;
> >> 	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY |
> >> 			    VM_FAULT_DONE_COW)))
> >> 		return ret;
> > 
> > Can you explain why you chose these two sites?  Rather than, for
> > example, way up in handle_mm_fault()?
> 
> I think they are the only two places that code can enter another filesystem

hm.  Maybe.  At this point in time.  I'm feeling that doing the
save/restore at the highest level is better.  It's cheap.

> > 
> > It's hard to believe that a fault handler will alter ->journal_info if
> > it is handling a read fault, so perhaps we only need to do this for a
> > write fault?  Although such an optimization probably isn't worthwhile. 
> > The whole thing is only about three instructions.
> 
> ceph uses current->journal_info for both read/write operations. I think btrfs also read current->journal_info during read-only operation. (I mentioned this in my previous reply)

Quite a lot of filesystems use ->journal_info.  Arguably it should be
the fs's responsibility to restore the old journal_info value after
having used it.  But that's a ton of changes :(