linux-kernel - Re: [PATCH v4 1/1] mm: introduce put_user

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Date:   Tue, 19 Mar 2019 17:29:18 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     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>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        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>,
        Tom Talpey <tom@...pey.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 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.

-- 
 Kirill A. Shutemov