From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f169.google.com (mail-wi0-f169.google.com [209.85.212.169]) by kanga.kvack.org (Postfix) with ESMTP id 9C6286B0038 for ; Tue, 11 Aug 2015 12:17:18 -0400 (EDT) Received: by wicne3 with SMTP id ne3so67878590wic.0 for ; Tue, 11 Aug 2015 09:17:18 -0700 (PDT) Received: from mail-wi0-f177.google.com (mail-wi0-f177.google.com. [209.85.212.177]) by mx.google.com with ESMTPS id d2si4533123wjw.157.2015.08.11.09.17.16 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 11 Aug 2015 09:17:16 -0700 (PDT) Received: by wicne3 with SMTP id ne3so67877331wic.0 for ; Tue, 11 Aug 2015 09:17:16 -0700 (PDT) Message-ID: <55CA2008.7070702@plexistor.com> Date: Tue, 11 Aug 2015 19:17:12 +0300 From: Boaz Harrosh MIME-Version: 1.0 Subject: Re: [PATCH, RFC 2/2] dax: use range_lock instead of i_mmap_lock References: <1439219664-88088-1-git-send-email-kirill.shutemov@linux.intel.com> <1439219664-88088-3-git-send-email-kirill.shutemov@linux.intel.com> <20150811081909.GD2650@quack.suse.cz> <20150811093708.GB906@dastard> <20150811135004.GC2659@quack.suse.cz> <55CA0728.7060001@plexistor.com> <20150811152850.GA2608@node.dhcp.inet.fi> In-Reply-To: <20150811152850.GA2608@node.dhcp.inet.fi> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: Jan Kara , Dave Chinner , "Kirill A. Shutemov" , Andrew Morton , Matthew Wilcox , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Davidlohr Bueso , Theodore Ts'o On 08/11/2015 06:28 PM, Kirill A. Shutemov wrote: <> >> Hi Jan. So you got me confused above. You say: >> "DAX which needs exclusive access to the page given range in the page cache" >> >> but DAX and page-cache are mutually exclusive. I guess you meant the VMA >> range, or the inode->mapping range (which one is it) > > The second -- pgoff range within the inode->mapping. > So yes this is what I do not understand with DAX the inode->mapping radix-tree is empty. >> Actually I do not understand this race you guys found at all. (Please bear with >> me sorry for being slow) >> >> If two threads of the same VMA fault on the same pte >> (I'm not sure how you call it I mean a single 4k entry at each VMAs page-table) >> then the mm knows how to handle this just fine. > > It does. But only if we have struct page. See lock_page_or_retry() in > filemap_fault(). Without lock_page() it's problematic. > >> If two processes, ie two VMAs fault on the same inode->mapping. Then an inode >> wide lock like XFS's to protect against i_size-change / truncate is more than >> enough. > > We also used lock_page() to make sure we shoot out all pages as we don't > exclude page faults during truncate. Consider this race: > > > get_block > check i_size > update i_size > unmap > setup pte > Please consider this senario then: read_lock(inode) get_block check i_size read_unlock(inode) write_lock(inode) update i_size * remove allocated blocks unmap write_unlock(inode) setup pte IS what you suppose to do in xfs > With normal page cache we make sure that all pages beyond i_size is > dropped using lock_page() in truncate_inode_pages_range(). > Yes there is no truncate_inode_pages_range() in DAX again radix tree is empty. Please do you have a reproducer I would like to see this race and also experiment with xfs (I guess you saw it in ext4) > For DAX we need a way to stop all page faults to the pgoff range before > doing unmap. > Why ? >> Because with DAX there is no inode->mapping "mapping" at all. You have the call >> into the FS with get_block() to replace "holes" (zero pages) with real allocated >> blocks, on WRITE faults, but this conversion should be protected inside the FS >> already. Then there is the atomic exchange of the PTE which is fine. >> (And vis versa with holes mapping and writes) > > Having unmap_mapping_range() in PMD fault handling is very unfortunate. > Go to rmap just to solve page fault is very wrong. > BTW, we need to do it in write path too. > Only the write path and only when we exchange a zero-page (hole) with a new allocated (written to) page. Both write fault and/or write-path > I'm not convinced that all these "let's avoid backing storage allocation" > in DAX code is not layering violation. I think the right place to solve > this is filesystem. And we have almost all required handles for this in > place. We only need to change vm_ops->page_mkwrite() interface to be able > to return different page than what was given on input. > What? there is no page returned for DAX page_mkwrite(), it is all insert_mixed with direct pmd. Ha I think I see what you are tumbling on. Maybe it is the zero-pages when read-mapping holes. A solution I have, (And is working for a year now) is have only a single zero-page per inode->mapping, inserted at all the holes. and again radix-tree is kept clean always. This both saves memory and avoids the race on the (always empty) radix tree. Tell me if you want that I send a patch there is a small trick I do at vm_ops->page_mkwrite(): /* our zero page doesn't really hold the correct offset to the file in * page->index so vmf->pgoff is incorrect, lets fix that */ vmf->pgoff = vma->vm_pgoff + (((unsigned long)vmf->virtual_address - vma->vm_start) >> PAGE_SHIFT); /* call fault handler to get a real page for writing */ return __page_fault(vma, vmf); Again the return from page_mkwrite() && __page_fault(WRITE_CASE) is always VM_FAULT_NOPAGE, right? >>> So regardless whether the lock will be a fs-private one or in >>> address_space, DAX needs something like the range lock Kirill suggested. >>> Having the range lock in fs-private part of inode has the advantage that >>> only filesystems supporting DAX / punch hole will pay the memory overhead. >>> OTOH most major filesystems need it so the savings would be IMO noticeable >> >> punch-hole is truncate for me. With the xfs model of read-write lock where >> truncate takes write, any fault taking read before executing the fault looks >> good for the FS side of things. I guess you mean the optimization of the >> radix-tree lock. But you see DAX does not have a radix-tree, ie it is empty. > > Hm. Where does XFS take this read-write lock in fault path? > > IIUC, truncation vs. page fault serialization relies on i_size being > updated before doing truncate_pagecache() and checking i_size under > page_lock() on fault side. We don't have i_size fence for punch hole. > again truncate_pagecache() is NONE. And yes the read-write locking will protect punch-hole just as truncate see my locking senario above. > BTW, how things like ext4_collapse_range() can be safe wrt parallel page > fault? Ted? > Thanks Boaz -- 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