netdev - Re: [PATCH v2] mm: page_frag: Warn_on when frag_alloc size is bigger than PAGE

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Date:   Tue, 31 May 2022 08:47:04 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Chen Lin <chen45464546@....com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, alexander.duyck@...il.com,
        netdev@...r.kernel.org
Subject: Re: [PATCH v2] mm: page_frag: Warn_on when frag_alloc size is
 bigger than PAGE_SIZE

On Tue, 31 May 2022 23:36:22 +0800 Chen Lin wrote:
> At 2022-05-31 22:14:12, "Jakub Kicinski" <kuba@...nel.org> wrote:
> >On Tue, 31 May 2022 22:41:12 +0800 Chen Lin wrote:  
> >> The sample code above cannot completely solve the current problem.
> >> For example, when fragsz is greater than PAGE_FRAG_CACHE_MAX_SIZE(32768),
> >> __page_frag_cache_refill will return a memory of only 32768 bytes, so 
> >> should we continue to expand the PAGE_FRAG_CACHE_MAX_SIZE? Maybe more 
> >> work needs to be done  
> >
> >Right, but I can think of two drivers off the top of my head which will
> >allocate <=32k frags but none which will allocate more.  
> 
> In fact, it is rare to apply for more than one page, so is it necessary to 
> change it to support? 

I don't really care if it's supported TBH, but I dislike adding 
a branch to the fast path just to catch one or two esoteric bad 
callers.

Maybe you can wrap the check with some debug CONFIG_ so it won't
run on production builds?

> we can just warning and return, also it is easy to synchronize this simple 
> protective measures to lower Linux versions.