linux-kernel - Re: [PATCH v5 3/3] mm/khugepaged: maintain page cache uptodate flag

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Date:   Wed, 29 Mar 2023 16:53:58 +0000
From:   Song Liu <songliubraving@...a.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Song Liu <songliubraving@...a.com>,
        Hugh Dickins <hughd@...gle.com>, Song Liu <song@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        David Stevens <stevensd@...omium.org>,
        Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Yang Shi <shy828301@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Jiaqi Yan <jiaqiyan@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 3/3] mm/khugepaged: maintain page cache uptodate flag



> On Mar 24, 2023, at 6:31 AM, Matthew Wilcox <willy@...radead.org> wrote:
> 
> On Fri, Mar 24, 2023 at 06:03:37AM +0000, Song Liu wrote:
>> 
>> 
>>> On Mar 23, 2023, at 8:30 PM, Matthew Wilcox <willy@...radead.org> wrote:
>> 
>> [...]
>> 
>>> 
>>> The Uptodate flag check needs to be done by the caller; the
>>> find_get_page() family return !uptodate pages.
>>> 
>>> But find_get_page() does not advertise itself as NMI-safe.  And I
>>> think it's wrong to try to make it NMI-safe.  Most of the kernel is
>>> not NMI-safe.  I think it's incumbent on the BPF people to get the
>>> information they need ahead of taking the NMI.  NMI handlers are not
>>> supposed to be doing a huge amount of work!  I don't really understand
>>> why it needs to do work in NMI context; surely it can note the location of
>>> the fault and queue work to be done later (eg on irq-enable, task-switch
>>> or return-to-user)
>> 
>> The use case here is a profiler (similar to perf-record). Parsing the 
>> build id in side the NMI makes the profiler a lot simpler. Otherwise, 
>> we will need some post processing for each sample. 
> 
> Simpler for you, maybe.  But this is an NMI!  It's not supposed to
> be doing printf-formatting or whatever, much less poking around
> in the file cache.  Like perf, it should record a sample and then
> convert that later.  Maybe it can defer to a tasklet, but i think
> scheduling work is a better option.
> 
>> OTOH, it is totally fine if build_id_parse() fails some time, say < 5%. 
>> The profiler output is still useful in such cases. 
>> 
>> I guess the next step is to replace find_get_page() with a NMI-safe
>> version?
> 
> No, absolutely not.  Stop doing so much work in an NMI.

While I understand the concern, it is not something we can easily remove, 
as there are users rely on this feature. How about we discuss this at
upcoming LSFMMBPF?

Thanks,
Song