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

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Date:   Thu, 30 Apr 2020 20:12:00 +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
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 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).


> 
> It adds a page flag ("daccess") which starts out set.  It clears the
> flag it when the page is added to the page cache or mapped as anonymous.
>  This are presumably the the two mostly likely kinds of pages to be
> problematic.  It re-sets the flag when it hits the new hook for s390:
> arch_make_page_accessible().
> 
> It then patches the DMA mapping API.  If a page gets to the DMA mapping
> API without being accessible, it hits a tracepoint.
> 
> It goes boom shortly after hitting userspace underneath a sys_sendto().
>  That code uses lib/iov_iter.c which does get_user_pages_fast() and
> apparently does not set FOLL_PIN, so never hits the s390 arch hooks.
> 
> I hacked out the FOLL_PIN check and just universally call the hook for
> all gup_pte_range() calls.  I think you'll need to do that as well.  I
> don't think the assumptions about FOLL_PIN always preceding I/O is true
> universally.  Hacking out FOLL_PIN quiets down the warning spew quite a
> bit, but it still hits a few of them.
> 
> Here's one example:
> 
>  0)  sd-reso-410   |               |  /* mm_accessible_error: ...
>       sd-resolve-410   [000] ....   212.918838: <stack trace>
>  => trace_event_raw_event_mm_accessible_error
>  => check_page_accessible
>  => e1000_xmit_frame
>  => dev_hard_start_xmit
>  => sch_direct_xmit
>  => __qdisc_run
>  => __dev_queue_xmit
>  => ip_finish_output2
>  => ip_output
>  => ip_send_skb
>  => udp_send_skb.isra.59
>  => udp_sendmsg
>  => ____sys_sendmsg
>  => ___sys_sendmsg
>  => __sys_sendmmsg
>  => __x64_sys_sendmmsg
>  => do_syscall_64
>  => entry_SYSCALL_64_after_hwframe
> 
> This is just from booting and sitting on an idle Ubuntu 16.04.6 system.
>  I think the process in question here is the systemd resolver.
>