From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Hugh Dickins <hughd@google.com>
Cc: Keno Fischer <keno@juliacomputing.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
torvalds@linux-foundation.org, gthelen@google.com,
npiggin@gmail.com, w@1wt.eu, oleg@redhat.com,
keescook@chromium.org, luto@kernel.org, mhocko@suse.com,
rientjes@google.com
Subject: Re: [PATCH] mm: Respect FOLL_FORCE/FOLL_COW for thp
Date: Wed, 11 Jan 2017 14:35:15 +0300 [thread overview]
Message-ID: <20170111113515.GB4895@node.shutemov.name> (raw)
In-Reply-To: <alpine.LSU.2.11.1701102300001.2996@eggly.anvils>
On Tue, Jan 10, 2017 at 11:06:10PM -0800, Hugh Dickins wrote:
> On Tue, 10 Jan 2017, Hugh Dickins wrote:
> > On Thu, 5 Jan 2017, Kirill A. Shutemov wrote:
> > > On Thu, Jan 05, 2017 at 12:36:58AM -0500, Keno Fischer wrote:
> > > > struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
> > > > pmd_t *pmd, int flags)
> > > > {
> > > > @@ -783,7 +793,7 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
> > > >
> > > > assert_spin_locked(pmd_lockptr(mm, pmd));
> > > >
> > > > - if (flags & FOLL_WRITE && !pmd_write(*pmd))
> > > > + if (flags & FOLL_WRITE && !can_follow_write_pmd(*pmd, flags))
> > > > return NULL;
> > >
> > > I don't think this part is needed: once we COW devmap PMD entry, we split
> > > it into PTE table, so IIUC we never get here with PMD.
> >
> > Hi Kirill,
> >
> > Would you mind double-checking that? You certainly know devmap
> > better than me, but I feel safer with Keno's original as above.
> >
> > I can see that fs/dax.c dax_iomap_pmd_fault() does
> >
> > /* Fall back to PTEs if we're going to COW */
> > if (write && !(vma->vm_flags & VM_SHARED))
> > goto fallback;
> >
> > But isn't there a case of O_RDWR fd, VM_SHARED PROT_READ mmap, and
> > FOLL_FORCE write to it, which does not COW (but relies on FOLL_COW)?
>
> And now I think I'm wrong, but please double-check even so: I think that
> case gets ruled out by the !is_cow_mapping(vm_flags) check in mm/gup.c,
> where we used to have a WARN_ON_ONCE() for a while.
Right, !is_cow_mapping(vm_flags) will filter the case out.
Also there's no way we will get FOLL_COW set for file THP (dax or not): we
never return VM_FAULT_WRITE there.
--
Kirill A. Shutemov
prev parent reply other threads:[~2017-01-11 11:35 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-05 5:36 [PATCH] mm: Respect FOLL_FORCE/FOLL_COW for thp Keno Fischer
2017-01-05 15:05 ` Kirill A. Shutemov
2017-01-05 21:06 ` Keno Fischer
2017-01-11 5:41 ` Hugh Dickins
2017-01-11 7:06 ` Hugh Dickins
2017-01-11 11:35 ` Kirill A. Shutemov [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20170111113515.GB4895@node.shutemov.name \
--to=kirill@shutemov.name \
--cc=gthelen@google.com \
--cc=hughd@google.com \
--cc=keescook@chromium.org \
--cc=keno@juliacomputing.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=luto@kernel.org \
--cc=mhocko@suse.com \
--cc=npiggin@gmail.com \
--cc=oleg@redhat.com \
--cc=rientjes@google.com \
--cc=torvalds@linux-foundation.org \
--cc=w@1wt.eu \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).