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

From: Song Liu <songliubraving@meta.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Song Liu <songliubraving@meta.com>,
	Hugh Dickins <hughd@google.com>, Song Liu <song@kernel.org>,
	Jiri Olsa <jolsa@kernel.org>,
	David Stevens <stevensd@chromium.org>,
	Linux-MM <linux-mm@kvack.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Peter Xu <peterx@redhat.com>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	Yang Shi <shy828301@gmail.com>,
	David Hildenbrand <david@redhat.com>,
	Jiaqi Yan <jiaqiyan@google.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v5 3/3] mm/khugepaged: maintain page cache uptodate flag
Date: Wed, 29 Mar 2023 16:53:58 +0000	[thread overview]
Message-ID: <E1A7D23D-EFCB-482F-9D9D-F8D92F1630D2@fb.com> (raw)
In-Reply-To: <ZB2mH/XMj3vh1RrN@casper.infradead.org>

> On Mar 24, 2023, at 6:31 AM, Matthew Wilcox <willy@infradead.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@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. 
> 
> 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