Messages in this thread | | | Subject | Re: [PATCH -v6 2/2] Updating ctime and mtime for memory-mapped files | From | Peter Zijlstra <> | Date | Fri, 18 Jan 2008 11:15:36 +0100 |
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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..
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