linux-kernel - Re: [PATCH v4 0/7] mm/mprotect: Fix dax puds

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Message-ID: <ZrPrYdmyUDRdz1QN@x1n>
Date: Wed, 7 Aug 2024 17:47:13 -0400
From: Peter Xu <peterx@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	"Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Oscar Salvador <osalvador@...e.de>,
	Dan Williams <dan.j.williams@...el.com>,
	James Houghton <jthoughton@...gle.com>,
	Matthew Wilcox <willy@...radead.org>,
	Nicholas Piggin <npiggin@...il.com>,
	Rik van Riel <riel@...riel.com>, Dave Jiang <dave.jiang@...el.com>,
	x86@...nel.org, Ingo Molnar <mingo@...hat.com>,
	Rick P Edgecombe <rick.p.edgecombe@...el.com>,
	"Kirill A . Shutemov" <kirill@...temov.name>,
	linuxppc-dev@...ts.ozlabs.org,
	Mel Gorman <mgorman@...hsingularity.net>,
	Hugh Dickins <hughd@...gle.com>, Borislav Petkov <bp@...en8.de>,
	David Hildenbrand <david@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vlastimil Babka <vbabka@...e.cz>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Huang Ying <ying.huang@...el.com>
Subject: Re: [PATCH v4 0/7] mm/mprotect: Fix dax puds

On Wed, Aug 07, 2024 at 02:23:16PM -0700, Andrew Morton wrote:
> On Wed,  7 Aug 2024 15:48:04 -0400 Peter Xu <peterx@...hat.com> wrote:
> 
> > 
> > Tests
> > =====
> > 
> > What I did test:
> > 
> > - cross-build tests that I normally cover [1]
> > 
> > - smoke tested on x86_64 the simplest program [2] on dev_dax 1G PUD
> >   mprotect() using QEMU's nvdimm emulations [3] and ndctl to create
> >   namespaces with proper alignments, which used to throw "bad pud" but now
> >   it'll run through all fine.  I checked sigbus happens if with illegal
> >   access on protected puds.
> > 
> > - vmtests.
> > 
> > What I didn't test:
> > 
> > - fsdax: I wanted to also give it a shot, but only until then I noticed it
> >   doesn't seem to be supported (according to dax_iomap_fault(), which will
> >   always fallback on PUD_ORDER).  I did remember it was supported before, I
> >   could miss something important there.. please shoot if so.
> 
> OK.  Who are you addressing this question to?

Anyone who is familiar with fsdax + 1g.  Maybe Matthew would be the most
suitable, but I didn't track further on fsdax.

> 
> > - userfault wp-async: I also wanted to test userfault-wp async be able to
> >   split huge puds (here it's simply a clear_pud.. though), but it won't
> >   work for devdax anyway due to not allowed to do smaller than 1G faults in
> >   this case. So skip too.
> 
> Sounds OK.  So that's an additional project if anyone cares enough?

Right.

> 
> > - Power, as no hardware on hand.
> 
> Hopefully the powerpc people can help with that.  What tests do you ask
> that they run?

The test program [2] in cover letter should work as a very basic test; one
needs to setup the dax device to use 1g mapping first, though:

[2] https://github.com/xzpeter/clibs/blob/master/misc/dax.c

At least per my experience not much fancy things we can do there, e.g., I
think at least dev_dax has a limitation on vma split that it must be 1g
aligned when use 1g mappings, so even split can't happen (as iirc I used to
try some random mprotect on smaller ranges)..

Thanks,

-- 
Peter Xu