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From: Peter Xu <peterx@redhat.com>
To: Hugh Dickins <hughd@google.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jerome Glisse <jglisse@redhat.com>, Joe Perches <joe@perches.com>,
	Lokesh Gidra <lokeshgidra@google.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>, Shaohua Li <shli@fb.com>,
	Shuah Khan <shuah@kernel.org>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	Wang Qing <wangqing@vivo.com>,
	linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	linux-mm@kvack.org, Brian Geffon <bgeffon@google.com>,
	"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
	Mina Almasry <almasrymina@google.com>,
	Oliver Upton <oupton@google.com>
Subject: Re: [PATCH v2 3/9] userfaultfd/shmem: support minor fault registration for shmem
Date: Wed, 14 Apr 2021 14:51:37 -0400
Message-ID: <20210414185137.GK4440@xz-x1> (raw)
In-Reply-To: <alpine.LSU.2.11.2104132351350.9086@eggly.anvils>

On Wed, Apr 14, 2021 at 12:36:13AM -0700, Hugh Dickins wrote:
> On Mon, 12 Apr 2021, Axel Rasmussen wrote:
> 
> > This patch allows shmem-backed VMAs to be registered for minor faults.
> > Minor faults are appropriately relayed to userspace in the fault path,
> > for VMAs with the relevant flag.
> > 
> > This commit doesn't hook up the UFFDIO_CONTINUE ioctl for shmem-backed
> > minor faults, though, so userspace doesn't yet have a way to resolve
> > such faults.
> 
> This is a very odd way to divide up the series: an "Intermission"
> half way through the implementation of MINOR/CONTINUE: this 3/9
> makes little sense without the 4/9 to mm/userfaultfd.c which follows.
> 
> But, having said that, I won't object and Peter did not object, and
> I don't know of anyone else looking here: it will only give each of
> us more trouble to insist on repartitioning the series, and it's the
> end state that's far more important to me and to all of us.

Agreed, ideally it should be after patch 4 since this patch enables the
feature already.

> 
> And I'll even seize on it, to give myself an intermission after
> this one, until tomorrow (when I'll look at 4/9 and 9/9 - but
> shall not look at the selftests ones at all).
> 
> Most of this is okay, except the mm/shmem.c part; and I've just now
> realized that somewhere (whether in this patch or separately) there
> needs to be an update to Documentation/admin-guide/mm/userfaultfd.rst
> (admin-guide? how weird, but not this series' business to correct).

(maybe some dir "devel" would suite better?  But I do also see soft-dirty.rst,
 idle_page_tracking.rst,..)

[...]

> >  static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
> > @@ -1820,6 +1820,14 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
> >  
> >  	page = pagecache_get_page(mapping, index,
> >  					FGP_ENTRY | FGP_HEAD | FGP_LOCK, 0);
> > +
> > +	if (page && vma && userfaultfd_minor(vma)) {
> > +		unlock_page(page);
> > +		put_page(page);
> > +		*fault_type = handle_userfault(vmf, VM_UFFD_MINOR);
> > +		return 0;
> > +	}
> > +
> 
> Okay, Peter persuaded you to move that up here: where indeed it
> does look better than the earlier "swapped" version.
> 
> But will crash on swap as it's currently written: it needs to say
> 		if (!xa_is_value(page)) {
> 			unlock_page(page);
> 			put_page(page);
> 		}

And this is definitely true...  Thanks,

> 
> I did say before that it's more robust to return from the swap
> case after doing the shmem_swapin_page(). But I might be slowly
> realizing that the ioctl to add the pte (in 4/9) will do its
> shmem_getpage_gfp(), and that will bring in the swap if user
> did not already do so: so I was wrong to claim more robustness
> the other way, this placement should be fine. I think.
> 
> >  	if (xa_is_value(page)) {
> >  		error = shmem_swapin_page(inode, index, &page,
> >  					  sgp, gfp, vma, fault_type);
> > -- 
> > 2.31.1.295.g9ea45b61b8-goog
> 

-- 
Peter Xu


  reply index

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-13  5:17 [PATCH v2 0/9] userfaultfd: add minor fault handling " Axel Rasmussen
2021-04-13  5:17 ` [PATCH v2 1/9] userfaultfd/hugetlbfs: avoid including userfaultfd_k.h in hugetlb.h Axel Rasmussen
2021-04-14  6:43   ` Hugh Dickins
2021-04-13  5:17 ` [PATCH v2 2/9] userfaultfd/shmem: combine shmem_{mcopy_atomic,mfill_zeropage}_pte Axel Rasmussen
2021-04-14  6:51   ` Hugh Dickins
2021-04-13  5:17 ` [PATCH v2 3/9] userfaultfd/shmem: support minor fault registration for shmem Axel Rasmussen
2021-04-13 20:43   ` Peter Xu
2021-04-14  7:36   ` Hugh Dickins
2021-04-14 18:51     ` Peter Xu [this message]
2021-04-13  5:17 ` [PATCH v2 4/9] userfaultfd/shmem: support UFFDIO_CONTINUE " Axel Rasmussen
2021-04-16 23:47   ` Hugh Dickins
2021-04-20 18:05     ` Axel Rasmussen
2021-04-13  5:17 ` [PATCH v2 5/9] userfaultfd/selftests: use memfd_create for shmem test type Axel Rasmussen
2021-04-13 20:16   ` Peter Xu
2021-04-13  5:17 ` [PATCH v2 6/9] userfaultfd/selftests: create alias mappings in the shmem test Axel Rasmussen
2021-04-13 20:17   ` Peter Xu
2021-04-13  5:17 ` [PATCH v2 7/9] userfaultfd/selftests: reinitialize test context in each test Axel Rasmussen
2021-04-13 20:15   ` Peter Xu
2021-04-15 18:03     ` Axel Rasmussen
2021-04-13  5:17 ` [PATCH v2 8/9] userfaultfd/selftests: exercise minor fault handling shmem support Axel Rasmussen
2021-04-13  5:17 ` [PATCH v2 9/9] userfaultfd/shmem: modify shmem_mcopy_atomic_pte to use install_ptes Axel Rasmussen
2021-04-17  0:34   ` Hugh Dickins
2021-04-20 18:43     ` Axel Rasmussen

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