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Date: Fri, 18 Jan 2008 13:39:58 +0300 From: "Anton Salikhmetov" <salikhmetov@...il.com> To: "Peter Zijlstra" <peterz@...radead.org> Cc: "Miklos Szeredi" <miklos@...redi.hu>, linux-mm@...ck.org, jakob@...hought.net, linux-kernel@...r.kernel.org, valdis.kletnieks@...edu, riel@...hat.com, ksm@...dk, staubach@...hat.com, jesper.juhl@...il.com, torvalds@...ux-foundation.org, akpm@...ux-foundation.org, protasnb@...il.com, r.e.wolff@...wizard.nl, hidave.darkstar@...il.com, hch@...radead.org Subject: Re: [PATCH -v6 2/2] Updating ctime and mtime for memory-mapped files 2008/1/18, Peter Zijlstra <peterz@...radead.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. > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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