From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f72.google.com (mail-wm0-f72.google.com [74.125.82.72]) by kanga.kvack.org (Postfix) with ESMTP id 4AB016B04A0 for ; Wed, 3 Jan 2018 19:17:57 -0500 (EST) Received: by mail-wm0-f72.google.com with SMTP id k126so158933wmd.5 for ; Wed, 03 Jan 2018 16:17:57 -0800 (PST) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTPS id 69si1608845wrk.16.2018.01.03.16.17.55 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 03 Jan 2018 16:17:56 -0800 (PST) Date: Wed, 3 Jan 2018 16:17:53 -0800 From: Andrew Morton Subject: Re: [PATCH] mm/fadvise: discard partial pages iff endbyte is also eof Message-Id: <20180103161753.8b22d32d640f6e0be4119081@linux-foundation.org> In-Reply-To: <20180103104800.xgqe32hv63xsmsjh@techsingularity.net> References: <1514002568-120457-1-git-send-email-shidao.ytt@alibaba-inc.com> <8DAEE48B-AD5D-4702-AB4B-7102DD837071@alibaba-inc.com> <20180103104800.xgqe32hv63xsmsjh@techsingularity.net> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Mel Gorman Cc: "??????(Caspar)" , green@linuxhacker.ru, linux-mm@kvack.org, linux-kernel@vger.kernel.org, "??????(??????)" , ?????? On Wed, 3 Jan 2018 10:48:00 +0000 Mel Gorman wrote: > On Wed, Jan 03, 2018 at 02:53:43PM +0800, ??????(Caspar) wrote: > > > > > > > ?? 2017??12??23????12:16?????? ?????? > > > > > > From: "shidao.ytt" > > > > > > in commit 441c228f817f7 ("mm: fadvise: document the > > > fadvise(FADV_DONTNEED) behaviour for partial pages") Mel Gorman > > > explained why partial pages should be preserved instead of discarded > > > when using fadvise(FADV_DONTNEED), however the actual codes to calcuate > > > end_index was unexpectedly wrong, the code behavior didn't match to the > > > statement in comments; Luckily in another commit 18aba41cbf > > > ("mm/fadvise.c: do not discard partial pages with POSIX_FADV_DONTNEED") > > > Oleg Drokin fixed this behavior > > > > > > Here I come up with a new idea that actually we can still discard the > > > last parital page iff the page-unaligned endbyte is also the end of > > > file, since no one else will use the rest of the page and it should be > > > safe enough to discard. > > > > +akpm... > > > > Hi Mel, Andrew: > > > > Would you please take a look at this patch, to see if this proposal > > is reasonable enough, thanks in advance! > > > > I'm backlogged after being out for the Christmas. Superficially the patch > looks ok but I wondered how often it happened in practice as we already > would discard files smaller than a page on DONTNEED. It also requires > that the system call get the exact size of the file correct and would not > discard if the off + len was past the end of the file for whatever reason > (e.g. a stat to read the size, a truncate in parallel and fadvise using > stale data from stat) and that's why the patch looked like it might have > no impact in practice. Is the patch known to help a real workload or is > it motivated by a code inspection? The current whole-pages-only logic was introduced (accidentally, I think) by yours truly when fixing a bug in the initial fadvise() commit in 2003. https://kernel.opensuse.org/cgit/kernel/commit/?h=v2.6.0-test4&id=7161ee20fea6e25a32feb91503ca2b7c7333c886 Namely: : invalidate_mapping_pages() takes start/end, but fadvise is currently passing : it start/len. : : : : mm/fadvise.c | 8 ++++++-- : 1 files changed, 6 insertions(+), 2 deletions(-) : : diff -puN mm/fadvise.c~fadvise-fix mm/fadvise.c : --- 25/mm/fadvise.c~fadvise-fix 2003-08-14 18:16:12.000000000 -0700 : +++ 25-akpm/mm/fadvise.c 2003-08-14 18:16:12.000000000 -0700 : @@ -26,6 +26,8 @@ long sys_fadvise64(int fd, loff_t offset : struct inode *inode; : struct address_space *mapping; : struct backing_dev_info *bdi; : + pgoff_t start_index; : + pgoff_t end_index; : int ret = 0; : : if (!file) : @@ -65,8 +67,10 @@ long sys_fadvise64(int fd, loff_t offset : case POSIX_FADV_DONTNEED: : if (!bdi_write_congested(mapping->backing_dev_info)) : filemap_flush(mapping); : - invalidate_mapping_pages(mapping, offset >> PAGE_CACHE_SHIFT, : - (len >> PAGE_CACHE_SHIFT) + 1); : + start_index = offset >> PAGE_CACHE_SHIFT; : + end_index = (offset + len + PAGE_CACHE_SIZE - 1) >> : + PAGE_CACHE_SHIFT; : + invalidate_mapping_pages(mapping, start_index, end_index); : break; : default: : ret = -EINVAL; : So I'm not sure that the whole "don't discard partial pages" thing is well-founded and I see no reason why we cannot alter it. So, thinking caps on: why not just discard them? After all, that's what userspace asked us to do. -- 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