From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <45DDD55F.4060106@redhat.com> Date: Thu, 22 Feb 2007 12:39:43 -0500 From: Peter Staubach MIME-Version: 1.0 Subject: Re: [PATCH] update ctime and mtime for mmaped write References: <20070221202615.a0a167f4.akpm@linux-foundation.org> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Miklos Szeredi Cc: akpm@linux-foundation.org, hugh@veritas.com, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org List-ID: Miklos Szeredi wrote: >> On Wed, 21 Feb 2007 18:51:52 +0100 Miklos Szeredi wrote: >> >> >>> This patch makes writing to shared memory mappings update st_ctime and >>> st_mtime as defined by SUSv3: >>> >>> The st_ctime and st_mtime fields of a file that is mapped with >>> MAP_SHARED and PROT_WRITE shall be marked for update at some point >>> in the interval between a write reference to the mapped region and >>> the next call to msync() with MS_ASYNC or MS_SYNC for that portion >>> of the file by any process. If there is no such call and if the >>> underlying file is modified as a result of a write reference, then >>> these fields shall be marked for update at some time after the >>> write reference. >>> >>> A new address_space flag is introduced: AS_CMTIME. This is set each >>> time a page is dirtied through a userspace memory mapping. This >>> includes write accesses via get_user_pages(). >>> >>> Note, the flag is set unconditionally, even if the page is already >>> dirty. This is important, because the page might have been dirtied >>> earlier by a non-mmap write. >>> >>> This flag is checked in msync() and __fput(), and if set, the file >>> times are updated and the flag is cleared >>> >>> The flag is also cleared, if the time update is triggered by a normal >>> write. This is not mandated by the standard, but seems to be a sane >>> thing to do. >>> >> Why is the flag checked in __fput()? >> > > It's because of this bit in the standard: > > If there is no such call and if the underlying file is modified > as a result of a write reference, then these fields shall be > marked for update at some time after the write reference. > > It could be done in munmap/mremap, but it seemed more difficult to > track down all the places where the vma is removed. But yes, that may > be a nicer solution. It seems to me that, with this support, a file, which is mmap'd, modified, but never msync'd or munmap'd, will never get its mtime updated. Or did I miss that? I also don't see how an mmap'd block device will get its mtime updated either. Thanx... ps -- 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