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From:   Song Liu <songliubraving@meta.com>
To:     Matthew Wilcox <willy@infradead.org>
CC:     Hugh Dickins <hughd@google.com>, Song Liu <song@kernel.org>,
        Jiri Olsa <jolsa@kernel.org>,
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Subject: Re: [PATCH v5 3/3] mm/khugepaged: maintain page cache uptodate flag
Thread-Topic: [PATCH v5 3/3] mm/khugepaged: maintain page cache uptodate flag
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Date:   Fri, 24 Mar 2023 06:03:37 +0000
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> On Mar 23, 2023, at 8:30 PM, Matthew Wilcox <willy@infradead.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. 

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?

Thanks,
Song