linux-kernel - Re: [PATCH v1 1/1] fs/splice: add missing callback for inaccessible pages

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Message-ID: <f681d61d-c83b-1472-a52f-d5cb951676fd@de.ibm.com>
Date:   Thu, 30 Apr 2020 21:54:11 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        akpm@...ux-foundation.org, jack@...e.cz, kirill@...temov.name,
        John Hubbard <jhubbard@...dia.com>
Cc:     david@...hat.com, aarcange@...hat.com, linux-mm@...ck.org,
        frankja@...ux.ibm.com, sfr@...b.auug.org.au, jhubbard@...dia.com,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        peterz@...radead.org, sean.j.christopherson@...el.com
Subject: Re: [PATCH v1 1/1] fs/splice: add missing callback for inaccessible
 pages



On 30.04.20 21:02, Christian Borntraeger wrote:
> On 30.04.20 20:12, Christian Borntraeger wrote:
>>
>>
>> On 29.04.20 18:07, Dave Hansen wrote:
>>> On 4/28/20 3:50 PM, Claudio Imbrenda wrote:
>>>> If a page is inaccesible and it is used for things like sendfile, then
>>>> the content of the page is not always touched, and can be passed
>>>> directly to a driver, causing issues.
>>>>
>>>> This patch fixes the issue by adding a call to arch_make_page_accessible
>>>> in page_cache_pipe_buf_confirm; this fixes the issue.
>>>
>>> I spent about 5 minutes putting together a patch:
>>>
>>> 	https://sr71.net/~dave/intel/accessible.patch
>>
>> You only set the page flag for compound pages. that of course leaves a big pile
>> of pages marked a not accessible, thus explaining the sendto trace and all kind
>> of other random traces.
>>
>>
>> What do you see when you also do the  SetPageAccessible(page);
>> in the else page of prep_new_page (order == 0).
>> (I do get > 10000 of these non compound page allocs just during boot).
>>
> 
> And yes, I think you are right that we should call the callback also for !FOLL_PIN.


Thinking again about this I am no longer sure. Adding John Hubbard.

Documentation/core-api/pin_user_pages.rst says:
-------snip----------
Another way of thinking about these flags is as a progression of restrictions:
FOLL_GET is for struct page manipulation, without affecting the data that the
struct page refers to. FOLL_PIN is a *replacement* for FOLL_GET, and is for
short term pins on pages whose data *will* get accessed. As such, FOLL_PIN is
a "more severe" form of pinning. And finally, FOLL_LONGTERM is an even more
restrictive case that has FOLL_PIN as a prerequisite: this is for pages that
will be pinned longterm, and whose data will be accessed.
-------snip----------

So John,is it ok to give a page to an I/O device where the code has used gup 
with FOLL_GET (or gup fast without pup) or would you consider this a bug?