linux-kernel - Re: [PATCH] mm: Respect FOLL_FORCE/FOLL

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Date:   Tue, 10 Jan 2017 21:41:43 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
cc:     Keno Fischer <keno@...iacomputing.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        torvalds@...ux-foundation.org, gthelen@...gle.com,
        npiggin@...il.com, w@....eu, oleg@...hat.com,
        keescook@...omium.org, luto@...nel.org, mhocko@...e.com,
        rientjes@...gle.com, hughd@...gle.com
Subject: Re: [PATCH] mm: Respect FOLL_FORCE/FOLL_COW for thp

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)?

Hugh