From mboxrd@z Thu Jan 1 00:00:00 1970 Received: by wa-out-1112.google.com with SMTP id m33so1952590wag.8 for ; Fri, 18 Jan 2008 14:04:52 -0800 (PST) Message-ID: <4df4ef0c0801181404m186bb847sd556e031e908b0b6@mail.gmail.com> Date: Sat, 19 Jan 2008 01:04:50 +0300 From: "Anton Salikhmetov" Subject: Re: [PATCH -v6 2/2] Updating ctime and mtime for memory-mapped files In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Content-Disposition: inline References: <12006091182260-git-send-email-salikhmetov@gmail.com> <4df4ef0c0801181158s3f783beaqead3d7049d4d3fa7@mail.gmail.com> <4df4ef0c0801181303o6656832g8b63d2a119a86a9c@mail.gmail.com> Sender: owner-linux-mm@kvack.org Return-Path: To: Linus Torvalds Cc: Miklos Szeredi , peterz@infradead.org, linux-mm@kvack.org, jakob@unthought.net, linux-kernel@vger.kernel.org, valdis.kletnieks@vt.edu, riel@redhat.com, ksm@42.dk, staubach@redhat.com, jesper.juhl@gmail.com, akpm@linux-foundation.org, protasnb@gmail.com, r.e.wolff@bitwizard.nl, hidave.darkstar@gmail.com, hch@infradead.org List-ID: 2008/1/19, Linus Torvalds : > > > On Sat, 19 Jan 2008, Anton Salikhmetov wrote: > > > > Before using pte_wrprotect() the vma_wrprotect() routine uses the > > pte_offset_map_lock() macro to get the PTE and to acquire the ptl > > spinlock. Why did you say that this code was not SMP-safe? It should > > be atomic, I think. > > It's atomic WITH RESPECT TO OTHER PEOPLE WHO GET THE LOCK. > > Guess how much another x86 CPU cares when it sets the accessed bit in > hardware? Thank you very much for taking part in this discussion. Personally, it's very important to me. But I'm not sure that I understand which bit can be lost. Please let me explain. The logic for my vma_wrprotect() routine was taken from the page_check_address() function in mm/rmap.c. Here is a code snippet of the latter function: pgd = pgd_offset(mm, address); if (!pgd_present(*pgd)) return NULL; pud = pud_offset(pgd, address); if (!pud_present(*pud)) return NULL; pmd = pmd_offset(pud, address); if (!pmd_present(*pmd)) return NULL; pte = pte_offset_map(pmd, address); /* Make a quick check before getting the lock */ if (!pte_present(*pte)) { pte_unmap(pte); return NULL; } ptl = pte_lockptr(mm, pmd); spin_lock(ptl); if (pte_present(*pte) && page_to_pfn(page) == pte_pfn(*pte)) { *ptlp = ptl; return pte; } pte_unmap_unlock(pte, ptl); The page_check_address() function is called from the page_mkclean_one() routine as follows: pte = page_check_address(page, mm, address, &ptl); if (!pte) goto out; if (pte_dirty(*pte) || pte_write(*pte)) { pte_t entry; flush_cache_page(vma, address, pte_pfn(*pte)); entry = ptep_clear_flush(vma, address, pte); entry = pte_wrprotect(entry); entry = pte_mkclean(entry); set_pte_at(mm, address, pte, entry); ret = 1; } pte_unmap_unlock(pte, ptl); The write-protection of the PTE is done using the pte_wrprotect() entity. I intended to do the same during msync() with MS_ASYNC. I understand that I'm taking a risk of looking a complete idiot now, however I don't see any difference between the two situations. I presumed that the code in mm/rmap.c was absolutely correct, that's why I basically reused the design. > > > The POSIX standard requires the ctime and mtime stamps to be updated > > not later than at the second call to msync() with the MS_ASYNC flag. > > .. and that is no excuse for bad code. > > Linus > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org