Re: [PATCH 1/1] mm: pagemap: limit scan to virtual region being asked

[Shiraz Hashim]

On Wed, Jan 14, 2015 at 01:08:40AM +0000, Naoya Horiguchi wrote:
> On Tue, Jan 13, 2015 at 05:57:04PM +0530, Shiraz Hashim wrote:
> > pagemap_read scans through the virtual address space of a
> > task till it prepares 'count' pagemaps or it reaches end
> > of task.
> > 
> > This presents a problem when the page walk doesn't happen
> > for vma with VM_PFNMAP set. In which case walk is silently
> > skipped and no pagemap is prepare, in turn making
> > pagemap_read to scan through task end, even crossing beyond
> > 'count', landing into a different vma region. This leads to
> > wrong presentation of mappings for that vma.
> > 
> > Fix this by limiting end_vaddr to the end of the virtual
> > address region being scanned.
> > 
> > Signed-off-by: Shiraz Hashim <shashim@xxxxxxxxxxxxxx>
> 
> This patch works in some case, but there still seems a problem in
> another case.
> 
> Consider that we have two vmas within some narrow
> (PAGEMAP_WALK_SIZE) region.  One vma in lower address is VM_PFNMAP,
> and the other vma in higher address is not.  Then a single call of
> walk_page_range() skips the first vma and scans the second vma, but
> the pagemap record of the second vma will be stored on the wrong
> offset in the buffer, because we just skip vma(VM_PFNMAP) without
> calling any callbacks (within which add_to_pagemap() increments
> pm.pos).
> 
> So calling pte_hole() for vma(VM_PFNMAP) looks a better fix to me.
> 

Thanks. That makes sense, If you are okay, I can send following patch formally.

diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index ad83195..b16ea60 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -200,6 +200,11 @@ int walk_page_range(unsigned long addr, unsigned long end,
                        if ((vma->vm_start <= addr) &&
                            (vma->vm_flags & VM_PFNMAP)) {
                                next = vma->vm_end;
+                               if (walk->pte_hole)
+                                       err = walk->pte_hole(addr, next, walk);
+                               if (err)
+                                       break;
+
                                pgd = pgd_offset(walk->mm, next);
                                continue;
                        }

regards
Shiraz
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