Re: [PATCH 2/5] fs: charge pipe buffers to memcg

[Vladimir Davydov]

On Tue, Sep 29, 2015 at 03:57:11PM -0700, Andrew Morton wrote:
> On Sat, 26 Sep 2015 13:45:54 +0300 Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> wrote:
> 
> > Pipe buffers can be generated unrestrictedly by an unprivileged
> > userspace process, so they shouldn't go unaccounted.
> > 
> > ...
> >
> > --- a/fs/pipe.c
> > +++ b/fs/pipe.c
> > @@ -400,7 +400,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
> >  			int copied;
> >  
> >  			if (!page) {
> > -				page = alloc_page(GFP_HIGHUSER);
> > +				page = alloc_kmem_pages(GFP_HIGHUSER, 0);
> >  				if (unlikely(!page)) {
> >  					ret = ret ? : -ENOMEM;
> >  					break;
> 
> This seems broken.  We have a page buffer page which has a weird
> ->mapcount.  Now it gets stolen (generic_pipe_buf_steal()) and spliced
> into pagecache.  Then the page gets mmapped and MM starts playing with
> its ->_mapcount?

Right you are! I completely forgot of vmsplice case. Surely, we need to
uncharge the page in the ->steal method and clear its PageKmem. Will fix
that.

Thanks,
Vladimir
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