linux-kernel - Re: [PATCH 38/94] mm/gup: Add mm_populate

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Date:   Mon, 3 May 2021 15:53:58 +0000
From:   Liam Howlett <liam.howlett@...cle.com>
To:     Michel Lespinasse <michel@...pinasse.org>
CC:     "maple-tree@...ts.infradead.org" <maple-tree@...ts.infradead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Song Liu <songliubraving@...com>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Rik van Riel <riel@...riel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michel Lespinasse <walken.cr@...il.com>
Subject: Re: [PATCH 38/94] mm/gup: Add mm_populate_vma() for use when the vma
 is known

* Michel Lespinasse <michel@...pinasse.org> [210501 01:13]:
> On Wed, Apr 28, 2021 at 03:36:08PM +0000, Liam Howlett wrote:
> > When a vma is known, avoid calling mm_populate to search for the vma to
> > populate.
> > 
> > Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> > ---
> >  mm/gup.c      | 20 ++++++++++++++++++++
> >  mm/internal.h |  4 ++++
> >  2 files changed, 24 insertions(+)
> > 
> > diff --git a/mm/gup.c b/mm/gup.c
> > index c3a17b189064..48fe98ab0729 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -1468,6 +1468,26 @@ long populate_vma_page_range(struct vm_area_struct *vma,
> >  				NULL, NULL, locked);
> >  }
> >  
> > +/*
> > + * mm_populate_vma() - Populate a single range in a single vma.
> > + * @vma: The vma to populate.
> > + * @start: The start address to populate
> > + * @end: The end address to stop populating
> > + *
> > + * Note: Ignores errors.
> > + */
> > +void mm_populate_vma(struct vm_area_struct *vma, unsigned long start,
> > +		unsigned long end)
> > +{
> > +	struct mm_struct *mm = current->mm;
> > +	int locked = 1;
> > +
> > +	mmap_read_lock(mm);
> > +	populate_vma_page_range(vma, start, end, &locked);
> > +	if (locked)
> > +		mmap_read_unlock(mm);
> > +}
> > +
> 
> This seems like a nonsensical API at first glance - VMAs that are found
> in the vma tree might be modified, merged, split, or freed at any time
> if the mmap lock is not held, so the API can not be safely used. I think
> this applies to maple tree vmas just as much as it did for rbtree vmas ?

This is correct - it cannot be used without having the mmap_sem lock.
This is a new internal mm code API and is used to avoid callers that use
mm_populate() on a range that is known to be in a single VMA and already
have that VMA.  So instead of re-walking the tree to re-find the VMAs,
this function can be used with the known VMA and range.

It is used as described in patch 39 and 40 of this series.

Thanks,
Liam