Message-ID: <20190319204512.GB3096@redhat.com>
Date:   Tue, 19 Mar 2019 16:45:13 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Tom Talpey <tom@...pey.com>
Cc:     Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        john.hubbard@...il.com, Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm@...ck.org, Al Viro <viro@...iv.linux.org.uk>,
        Christian Benvenuti <benve@...co.com>,
        Christoph Hellwig <hch@...radead.org>,
        Christopher Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, John Hubbard <jhubbard@...dia.com>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH v4 1/1] mm: introduce put_user_page*(), placeholder
 versions

On Tue, Mar 19, 2019 at 03:43:44PM -0500, Tom Talpey wrote:
> On 3/19/2019 4:03 AM, Ira Weiny wrote:
> > On Tue, Mar 19, 2019 at 04:36:44PM +0100, Jan Kara wrote:
> > > On Tue 19-03-19 17:29:18, Kirill A. Shutemov wrote:
> > > > On Tue, Mar 19, 2019 at 10:14:16AM -0400, Jerome Glisse wrote:
> > > > > On Tue, Mar 19, 2019 at 09:47:24AM -0400, Jerome Glisse wrote:
> > > > > > On Tue, Mar 19, 2019 at 03:04:17PM +0300, Kirill A. Shutemov wrote:
> > > > > > > On Fri, Mar 08, 2019 at 01:36:33PM -0800, john.hubbard@...il.com wrote:
> > > > > > > > From: John Hubbard <jhubbard@...dia.com>
> > > > > > 
> > > > > > [...]
> > > > > > 
> > > > > > > > diff --git a/mm/gup.c b/mm/gup.c
> > > > > > > > index f84e22685aaa..37085b8163b1 100644
> > > > > > > > --- a/mm/gup.c
> > > > > > > > +++ b/mm/gup.c
> > > > > > > > @@ -28,6 +28,88 @@ struct follow_page_context {
> > > > > > > >   	unsigned int page_mask;
> > > > > > > >   };
> > > > > > > > +typedef int (*set_dirty_func_t)(struct page *page);
> > > > > > > > +
> > > > > > > > +static void __put_user_pages_dirty(struct page **pages,
> > > > > > > > +				   unsigned long npages,
> > > > > > > > +				   set_dirty_func_t sdf)
> > > > > > > > +{
> > > > > > > > +	unsigned long index;
> > > > > > > > +
> > > > > > > > +	for (index = 0; index < npages; index++) {
> > > > > > > > +		struct page *page = compound_head(pages[index]);
> > > > > > > > +
> > > > > > > > +		if (!PageDirty(page))
> > > > > > > > +			sdf(page);
> > > > > > > 
> > > > > > > How is this safe? What prevents the page to be cleared under you?
> > > > > > > 
> > > > > > > If it's safe to race clear_page_dirty*() it has to be stated explicitly
> > > > > > > with a reason why. It's not very clear to me as it is.
> > > > > > 
> > > > > > The PageDirty() optimization above is fine to race with clear the
> > > > > > page flag as it means it is racing after a page_mkclean() and the
> > > > > > GUP user is done with the page so page is about to be write back
> > > > > > ie if (!PageDirty(page)) see the page as dirty and skip the sdf()
> > > > > > call while a split second after TestClearPageDirty() happens then
> > > > > > it means the racing clear is about to write back the page so all
> > > > > > is fine (the page was dirty and it is being clear for write back).
> > > > > > 
> > > > > > If it does call the sdf() while racing with write back then we
> > > > > > just redirtied the page just like clear_page_dirty_for_io() would
> > > > > > do if page_mkclean() failed so nothing harmful will come of that
> > > > > > neither. Page stays dirty despite write back it just means that
> > > > > > the page might be write back twice in a row.
> > > > > 
> > > > > Forgot to mention one thing, we had a discussion with Andrea and Jan
> > > > > about set_page_dirty() and Andrea had the good idea of maybe doing
> > > > > the set_page_dirty() at GUP time (when GUP with write) not when the
> > > > > GUP user calls put_page(). We can do that by setting the dirty bit
> > > > > in the pte for instance. They are few bonus of doing things that way:
> > > > >      - amortize the cost of calling set_page_dirty() (ie one call for
> > > > >        GUP and page_mkclean()
> > > > >      - it is always safe to do so at GUP time (ie the pte has write
> > > > >        permission and thus the page is in correct state)
> > > > >      - safe from truncate race
> > > > >      - no need to ever lock the page
> > > > > 
> > > > > Extra bonus from my point of view, it simplify thing for my generic
> > > > > page protection patchset (KSM for file back page).
> > > > > 
> > > > > So maybe we should explore that ? It would also be a lot less code.
> > > > 
> > > > Yes, please. It sounds more sensible to me to dirty the page on get, not
> > > > on put.
> > > 
> > > I fully agree this is a desirable final state of affairs.
> > 
> > I'm glad to see this presented because it has crossed my mind more than once
> > that effectively a GUP pinned page should be considered "dirty" at all times
> > until the pin is removed.  This is especially true in the RDMA case.
> 
> But, what if the RDMA registration is readonly? That's not uncommon, and
> marking dirty unconditonally would add needless overhead to such pages.

Yes and this is only when FOLL_WRITE is set ie when you are doing GUP and
asking for write. Doing GUP and asking for read is always safe.

Cheers,
Jérôme

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