linux-kernel - Re: [PATCH v9 03/10] mm/rmap: Split try_to_munlock from try_to

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Date:   Fri, 4 Jun 2021 20:49:39 +0000
From:   Liam Howlett <liam.howlett@...cle.com>
To:     Shakeel Butt <shakeelb@...gle.com>
CC:     Alistair Popple <apopple@...dia.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>,
        "bskeggs@...hat.com" <bskeggs@...hat.com>,
        "rcampbell@...dia.com" <rcampbell@...dia.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "jhubbard@...dia.com" <jhubbard@...dia.com>,
        "bsingharora@...il.com" <bsingharora@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "jglisse@...hat.com" <jglisse@...hat.com>,
        "willy@...radead.org" <willy@...radead.org>,
        "jgg@...dia.com" <jgg@...dia.com>,
        "peterx@...hat.com" <peterx@...hat.com>,
        "hughd@...gle.com" <hughd@...gle.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v9 03/10] mm/rmap: Split try_to_munlock from try_to_unmap

* Shakeel Butt <shakeelb@...gle.com> [210525 19:45]:
> On Tue, May 25, 2021 at 11:40 AM Liam Howlett <liam.howlett@...cle.com> wrote:
> >
> [...]
> > >
> > > +/*
> > > + * Walks the vma's mapping a page and mlocks the page if any locked vma's are
> > > + * found. Once one is found the page is locked and the scan can be terminated.
> > > + */
> >
> > Can you please add that this requires the mmap_sem() lock to the
> > comments?
> >
> 
> Why does this require mmap_sem() lock? Also mmap_sem() lock of which mm_struct?


Doesn't the mlock_vma_page() require the mmap_sem() for reading?  The
mm_struct in vma->vm_mm;


>From what I can see, at least the following paths have mmap_lock held
for writing:

munlock_vma_pages_range() from __do_munmap()
munlokc_vma_pages_range() from remap_file_pages()

> 
> > > +static bool page_mlock_one(struct page *page, struct vm_area_struct *vma,
> > > +                              unsigned long address, void *unused)
> > > +{
> > > +     struct page_vma_mapped_walk pvmw = {
> > > +             .page = page,
> > > +             .vma = vma,
> > > +             .address = address,
> > > +     };
> > > +
> > > +     /* An un-locked vma doesn't have any pages to lock, continue the scan */
> > > +     if (!(vma->vm_flags & VM_LOCKED))
> > > +             return true;
> > > +
> > > +     while (page_vma_mapped_walk(&pvmw)) {
> > > +             /* PTE-mapped THP are never mlocked */
> > > +             if (!PageTransCompound(page))
> > > +                     mlock_vma_page(page);
> > > +             page_vma_mapped_walk_done(&pvmw);
> > > +
> > > +             /*
> > > +              * no need to continue scanning other vma's if the page has
> > > +              * been locked.
> > > +              */
> > > +             return false;
> > > +     }
> > > +
> > > +     return true;
> > > +}

munlock_vma_pages_range() comments still references try_to_{munlock|unmap}