Re: [PATCH -v6 2/2] Updating ctime and mtime for memory-mapped files

["Anton Salikhmetov" <salikhmetov@gmail.com>]

2008/1/18, Peter Zijlstra <peterz@infradead.org>:
>
> On Fri, 2008-01-18 at 11:15 +0100, Peter Zijlstra wrote:
> > On Fri, 2008-01-18 at 10:51 +0100, Miklos Szeredi wrote:
> >
> > > > diff --git a/mm/msync.c b/mm/msync.c
> > > > index a4de868..a49af28 100644
> > > > --- a/mm/msync.c
> > > > +++ b/mm/msync.c
> > > > @@ -13,11 +13,33 @@
> > > >  #include <linux/syscalls.h>
> > > >
> > > >  /*
> > > > + * Scan the PTEs for pages belonging to the VMA and mark them read-only.
> > > > + * It will force a pagefault on the next write access.
> > > > + */
> > > > +static void vma_wrprotect(struct vm_area_struct *vma)
> > > > +{
> > > > + unsigned long addr;
> > > > +
> > > > + for (addr = vma->vm_start; addr < vma->vm_end; addr += PAGE_SIZE) {
> > > > +         spinlock_t *ptl;
> > > > +         pgd_t *pgd = pgd_offset(vma->vm_mm, addr);
> > > > +         pud_t *pud = pud_offset(pgd, addr);
> > > > +         pmd_t *pmd = pmd_offset(pud, addr);
> > > > +         pte_t *pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> > > > +
> > > > +         if (pte_dirty(*pte) && pte_write(*pte))
> > > > +                 *pte = pte_wrprotect(*pte);
> > > > +         pte_unmap_unlock(pte, ptl);
> > > > + }
> > > > +}
> > >
> > > What about ram based filesystems?  They don't start out with read-only
> > > pte's, so I think they don't want them read-protected now either.
> > > Unless this is essential for correct mtime/ctime accounting on these
> > > filesystems (I don't think it really is).  But then the mapping should
> > > start out read-only as well, otherwise the time update will only work
> > > after an msync(MS_ASYNC).
> >
> > page_mkclean() has all the needed logic for this, it also walks the rmap
> > and cleans out all other users, which I think is needed too for
> > consistencies sake:
> >
> > Process A                     Process B
> >
> > mmap(foo.txt)                 mmap(foo.txt)
> >
> > dirty page
> >                               dirty page
> >
> > msync(MS_ASYNC)
> >
> >                               dirty page
> >
> > msync(MS_ASYNC) <--- now what?!
> >
> >
> > So what I would suggest is using the page table walkers from mm, and
> > walks the page range, obtain the page using vm_normal_page() and call
> > page_mkclean(). (Oh, and ensure you don't nest the pte lock :-)
> >
> > All in all, that sounds rather expensive..
>
> Bah, and will break on s390... so we'd need a page_mkclean() variant
> that doesn't actually clear dirty.

So the current version of the functional changes patch takes this into account.

>
>