linux-kernel - Re: [patch 9/9] mm: fix pagecache write deadlocks

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Date:	Sun, 4 Feb 2007 02:30:55 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux Filesystems <linux-fsdevel@...r.kernel.org>,
	Linux Memory Management <linux-mm@...ck.org>
Subject: Re: [patch 9/9] mm: fix pagecache write deadlocks

On Sun, 4 Feb 2007 11:15:29 +0100 Nick Piggin <npiggin@...e.de> wrote:

> On Sun, Feb 04, 2007 at 01:44:45AM -0800, Andrew Morton wrote:
> > On Sun,  4 Feb 2007 09:51:07 +0100 (CET) Nick Piggin <npiggin@...e.de> wrote:
> > 
> > > 2.  If we find the destination page is non uptodate, unlock it (this could be
> > >     made slightly more optimal), then find and pin the source page with
> > >     get_user_pages. Relock the destination page and continue with the copy.
> > >     However, instead of a usercopy (which might take a fault), copy the data
> > >     via the kernel address space.
> > 
> > argh.  We just can't go adding all this gunk into the write() path. 
> > 
> > mmap_sem, a full pte-walk, taking of pte-page locks, etc.  For every page. 
> > Even single-process write() will suffer, let along multithreaded stuff,
> > where mmap_sem contention may be the bigger problem.
> 
> The write path is broken. I prefer my kernels slow, than buggy.

That won't fly.

> > There's a build error in filemap_xip.c btw.

?

> > 
> > We need to think different.
> > 
> > What happened to the idea of doing an atomic copy into the non-uptodate
> > page and handling it somehow?
> 
> That was my second idea.

Coulda sworn it was mine ;) I thought you ended up deciding it wasn't
practical because of the games we needed to play with ->commit_write.

> I didn't get any feedback on that patchset
> except to try this method, so I assume everyone hated it.
> 
> I actually liked it, because it didn't have to do the writev
> segment-at-a-time for !uptodate pages like this one does. Considering
> this code gets called from mm-less contexts, maybe I'll have to go back
> to this approach.

OK.

> > Another option might be to effectively pin the whole mm during the copy:
> > 
> > 	down_read(&current->mm->unpaging_lock);
> > 	get_user(addr);		/* Fault the page in */
> > 	...
> > 	copy_from_user()
> > 	up_read(&current->mm->unpaging_lock);
> > 
> > then, anyone who wants to unmap pages from this mm requires
> > write_lock(unpaging_lock).  So we know the results of that get_user()
> > cannot be undone.
> 
> Fugly.

I invited you to think different - don't just fixate on one random
tossed-out-there suggestion.

> but you introduce the theoretical memory deadlock
> where a task cannot reclaim its own memory.

Nah, that'll never happen - both pages are already allocated.

It's better than taking mmap_sem and walking pagetables...
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