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Message-ID: <20170616194437.GA20742@linux.intel.com> Date: Fri, 16 Jun 2017 13:44:37 -0600 From: Ross Zwisler <ross.zwisler@...ux.intel.com> To: Jan Kara <jack@...e.cz> Cc: Ross Zwisler <ross.zwisler@...ux.intel.com>, Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, "Darrick J. Wong" <darrick.wong@...cle.com>, Theodore Ts'o <tytso@....edu>, Alexander Viro <viro@...iv.linux.org.uk>, Andreas Dilger <adilger.kernel@...ger.ca>, Christoph Hellwig <hch@....de>, Dan Williams <dan.j.williams@...el.com>, Dave Hansen <dave.hansen@...el.com>, Ingo Molnar <mingo@...hat.com>, Jonathan Corbet <corbet@....net>, Matthew Wilcox <mawilcox@...rosoft.com>, Steven Rostedt <rostedt@...dmis.org>, linux-doc@...r.kernel.org, linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, linux-nvdimm@...ts.01.org, linux-xfs@...r.kernel.org Subject: Re: [PATCH v2 1/3] mm: add vm_insert_mixed_mkwrite() On Thu, Jun 15, 2017 at 04:42:04PM +0200, Jan Kara wrote: > On Wed 14-06-17 11:22:09, Ross Zwisler wrote: > > To be able to use the common 4k zero page in DAX we need to have our PTE > > fault path look more like our PMD fault path where a PTE entry can be > > marked as dirty and writeable as it is first inserted, rather than waiting > > for a follow-up dax_pfn_mkwrite() => finish_mkwrite_fault() call. > > > > Right now we can rely on having a dax_pfn_mkwrite() call because we can > > distinguish between these two cases in do_wp_page(): > > > > case 1: 4k zero page => writable DAX storage > > case 2: read-only DAX storage => writeable DAX storage > > > > This distinction is made by via vm_normal_page(). vm_normal_page() returns > > false for the common 4k zero page, though, just as it does for DAX ptes. > > Instead of special casing the DAX + 4k zero page case, we will simplify our > > DAX PTE page fault sequence so that it matches our DAX PMD sequence, and > > get rid of dax_pfn_mkwrite() completely. > > > > This means that insert_pfn() needs to follow the lead of insert_pfn_pmd() > > and allow us to pass in a 'mkwrite' flag. If 'mkwrite' is set insert_pfn() > > will do the work that was previously done by wp_page_reuse() as part of the > > dax_pfn_mkwrite() call path. > > > > Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com> > > So I agree that getting rid of dax_pfn_mkwrite() and using fault handler in > that case is a way to go. However I somewhat dislike the > vm_insert_mixed_mkwrite() thing - it looks like a hack - and I'm aware that > we have a similar thing for PMD which is ugly as well. Besides being ugly > I'm also concerned that when 'mkwrite' is set, we just silently overwrite > whatever PTE was installed at that position. Not that I'd see how that > could screw us for DAX but still a concern that e.g. some PTE flag could > get discarded by this is there... In fact, for !HAVE_PTE_SPECIAL > architectures, you will leak zero page references by just overwriting the > PTE - for those archs you really need to unmap zero page before replacing > PTE (and the same for PMD I suppose). > > So how about some vmf_insert_pfn(vmf, pe_size, pfn) helper that would > properly detect PTE / PMD case, read / write case etc., check that PTE did > not change from orig_pte, and handle all the nasty details instead of > messing with insert_pfn? > > Honza Sounds good, I'll figure this out for v3. Thanks for the review!
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