From: Tom Talpey <tom@talpey.com>
To: Jerome Glisse <jglisse@redhat.com>
Cc: Ira Weiny <ira.weiny@intel.com>, Jan Kara <jack@suse.cz>,
"Kirill A. Shutemov" <kirill@shutemov.name>,
john.hubbard@gmail.com, Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, Al Viro <viro@zeniv.linux.org.uk>,
Christian Benvenuti <benve@cisco.com>,
Christoph Hellwig <hch@infradead.org>,
Christopher Lameter <cl@linux.com>,
Dan Williams <dan.j.williams@intel.com>,
Dave Chinner <david@fromorbit.com>,
Dennis Dalessandro <dennis.dalessandro@intel.com>,
Doug Ledford <dledford@redhat.com>,
Jason Gunthorpe <jgg@ziepe.ca>,
Matthew Wilcox <willy@infradead.org>,
Michal Hocko <mhocko@kernel.org>,
Mike Rapoport <rppt@linux.ibm.com>,
Mike Marciniszyn <mike.marciniszyn@intel.com>,
Ralph Campbell <rcampbell@nvidia.com>,
LKML <linux-kernel@vger.kernel.org>,
linux-fsdevel@vger.kernel.org, John Hubbard <jhubbard@nvidia.com>,
Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH v4 1/1] mm: introduce put_user_page*(), placeholder versions
Date: Tue, 19 Mar 2019 15:55:23 -0500 [thread overview]
Message-ID: <0bdce970-1ec4-6bda-b82a-015fa68535a3@talpey.com> (raw)
In-Reply-To: <20190319204512.GB3096@redhat.com>
On 3/19/2019 3:45 PM, Jerome Glisse wrote:
> On Tue, Mar 19, 2019 at 03:43:44PM -0500, Tom Talpey wrote:
>> On 3/19/2019 4:03 AM, Ira Weiny wrote:
>>> On Tue, Mar 19, 2019 at 04:36:44PM +0100, Jan Kara wrote:
>>>> On Tue 19-03-19 17:29:18, Kirill A. Shutemov wrote:
>>>>> On Tue, Mar 19, 2019 at 10:14:16AM -0400, Jerome Glisse wrote:
>>>>>> On Tue, Mar 19, 2019 at 09:47:24AM -0400, Jerome Glisse wrote:
>>>>>>> On Tue, Mar 19, 2019 at 03:04:17PM +0300, Kirill A. Shutemov wrote:
>>>>>>>> On Fri, Mar 08, 2019 at 01:36:33PM -0800, john.hubbard@gmail.com wrote:
>>>>>>>>> From: John Hubbard <jhubbard@nvidia.com>
>>>>>>>
>>>>>>> [...]
>>>>>>>
>>>>>>>>> diff --git a/mm/gup.c b/mm/gup.c
>>>>>>>>> index f84e22685aaa..37085b8163b1 100644
>>>>>>>>> --- a/mm/gup.c
>>>>>>>>> +++ b/mm/gup.c
>>>>>>>>> @@ -28,6 +28,88 @@ struct follow_page_context {
>>>>>>>>> unsigned int page_mask;
>>>>>>>>> };
>>>>>>>>> +typedef int (*set_dirty_func_t)(struct page *page);
>>>>>>>>> +
>>>>>>>>> +static void __put_user_pages_dirty(struct page **pages,
>>>>>>>>> + unsigned long npages,
>>>>>>>>> + set_dirty_func_t sdf)
>>>>>>>>> +{
>>>>>>>>> + unsigned long index;
>>>>>>>>> +
>>>>>>>>> + for (index = 0; index < npages; index++) {
>>>>>>>>> + struct page *page = compound_head(pages[index]);
>>>>>>>>> +
>>>>>>>>> + if (!PageDirty(page))
>>>>>>>>> + sdf(page);
>>>>>>>>
>>>>>>>> How is this safe? What prevents the page to be cleared under you?
>>>>>>>>
>>>>>>>> If it's safe to race clear_page_dirty*() it has to be stated explicitly
>>>>>>>> with a reason why. It's not very clear to me as it is.
>>>>>>>
>>>>>>> The PageDirty() optimization above is fine to race with clear the
>>>>>>> page flag as it means it is racing after a page_mkclean() and the
>>>>>>> GUP user is done with the page so page is about to be write back
>>>>>>> ie if (!PageDirty(page)) see the page as dirty and skip the sdf()
>>>>>>> call while a split second after TestClearPageDirty() happens then
>>>>>>> it means the racing clear is about to write back the page so all
>>>>>>> is fine (the page was dirty and it is being clear for write back).
>>>>>>>
>>>>>>> If it does call the sdf() while racing with write back then we
>>>>>>> just redirtied the page just like clear_page_dirty_for_io() would
>>>>>>> do if page_mkclean() failed so nothing harmful will come of that
>>>>>>> neither. Page stays dirty despite write back it just means that
>>>>>>> the page might be write back twice in a row.
>>>>>>
>>>>>> Forgot to mention one thing, we had a discussion with Andrea and Jan
>>>>>> about set_page_dirty() and Andrea had the good idea of maybe doing
>>>>>> the set_page_dirty() at GUP time (when GUP with write) not when the
>>>>>> GUP user calls put_page(). We can do that by setting the dirty bit
>>>>>> in the pte for instance. They are few bonus of doing things that way:
>>>>>> - amortize the cost of calling set_page_dirty() (ie one call for
>>>>>> GUP and page_mkclean()
>>>>>> - it is always safe to do so at GUP time (ie the pte has write
>>>>>> permission and thus the page is in correct state)
>>>>>> - safe from truncate race
>>>>>> - no need to ever lock the page
>>>>>>
>>>>>> Extra bonus from my point of view, it simplify thing for my generic
>>>>>> page protection patchset (KSM for file back page).
>>>>>>
>>>>>> So maybe we should explore that ? It would also be a lot less code.
>>>>>
>>>>> Yes, please. It sounds more sensible to me to dirty the page on get, not
>>>>> on put.
>>>>
>>>> I fully agree this is a desirable final state of affairs.
>>>
>>> I'm glad to see this presented because it has crossed my mind more than once
>>> that effectively a GUP pinned page should be considered "dirty" at all times
>>> until the pin is removed. This is especially true in the RDMA case.
>>
>> But, what if the RDMA registration is readonly? That's not uncommon, and
>> marking dirty unconditonally would add needless overhead to such pages.
>
> Yes and this is only when FOLL_WRITE is set ie when you are doing GUP and
> asking for write. Doing GUP and asking for read is always safe.
Aha, ok great.
I guess it does introduce something for callers to be aware of, if
they GUP very large regions. I suppose if they're sufficiently aware
of the situation, e.g. pnfs LAYOUT_COMMIT notifications, they could
walk lists and reset page_dirty for untouched pages before releasing.
That's their issue though, and agreed it's safest for the GUP layer
to mark.
Tom.
next prev parent reply other threads:[~2019-03-19 20:55 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-08 21:36 [PATCH v4 0/1] mm: introduce put_user_page*(), placeholder versions john.hubbard
2019-03-08 21:36 ` [PATCH v4 1/1] " john.hubbard
2019-03-19 12:04 ` Kirill A. Shutemov
2019-03-19 13:47 ` Jerome Glisse
2019-03-19 14:06 ` Kirill A. Shutemov
2019-03-19 14:15 ` Jerome Glisse
2019-03-19 20:01 ` John Hubbard
2019-03-20 9:28 ` Kirill A. Shutemov
2019-03-19 14:14 ` Jerome Glisse
2019-03-19 14:29 ` Kirill A. Shutemov
2019-03-19 15:36 ` Jan Kara
2019-03-19 9:03 ` Ira Weiny
2019-03-19 20:43 ` Tom Talpey
2019-03-19 20:45 ` Jerome Glisse
2019-03-19 20:55 ` Tom Talpey [this message]
2019-03-19 19:02 ` John Hubbard
2019-03-19 21:23 ` Dave Chinner
2019-03-19 22:06 ` Jerome Glisse
2019-03-19 23:57 ` Dave Chinner
2019-03-20 0:08 ` Jerome Glisse
2019-03-20 1:43 ` John Hubbard
2019-03-20 4:33 ` Jerome Glisse
2019-03-20 9:08 ` Ira Weiny
2019-03-20 14:55 ` William Kucharski
2019-03-20 14:59 ` Jerome Glisse
2019-03-20 0:15 ` John Hubbard
2019-03-20 1:01 ` Christopher Lameter
2019-03-19 19:24 ` John Hubbard
2019-03-20 9:40 ` Kirill A. Shutemov
2019-03-08 23:21 ` [PATCH v4 0/1] " John Hubbard
2019-03-19 18:12 ` Christopher Lameter
2019-03-19 19:24 ` John Hubbard
2019-03-20 1:09 ` Christopher Lameter
2019-03-20 1:18 ` John Hubbard
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