linux-kernel - Re: [PATCH] mm: append __GFP_COMP flag for trace

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Message-ID: <CAEVVKH8AFMqwEcpp=7h7k-_BYqwU+gutoUMNM93pnGPiW=u+gg@mail.gmail.com>
Date:   Wed, 28 Apr 2021 11:05:18 +0800
From:   Xiongwei Song <sxwjean@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Xiongwei Song <sxwjean@...com>, cl@...ux.com, penberg@...nel.org,
        rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, vbabka@...e.cz, linux-mm@...ck.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: append __GFP_COMP flag for trace_malloc

On Tue, Apr 27, 2021 at 7:26 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Tue, Apr 27, 2021 at 01:30:48PM +0800, Xiongwei Song wrote:
> > Hi Mattew,
> >
> > One more thing I should explain, the kmalloc_order() appends the
> > __GFP_COMP flags,
> > not by the caller.
> >
> > void *kmalloc_order(size_t size, gfp_t flags, unsigned int order)
> > {
> > ...........................................................
> >
> > flags |= __GFP_COMP;
> > page = alloc_pages(flags, order);
> > ...........................................................
> > return ret;
> > }
> > EXPORT_SYMBOL(kmalloc_order);
> >
> > #ifdef CONFIG_TRACING
> > void *kmalloc_order_trace(size_t size, gfp_t flags, unsigned int order)
> > {
> > void *ret = kmalloc_order(size, flags, order);
> > trace_kmalloc(_RET_IP_, ret, size, PAGE_SIZE << order, flags);
> > return ret;
> > }
> > EXPORT_SYMBOL(kmalloc_order_trace);
> > #endif
>
> Yes, I understood that.  What I don't understand is why appending the
> __GFP_COMP to the trace would have been less confusing for you.
>
> Suppose I have some code which calls:
>
>         kmalloc(10 * 1024, GFP_ATOMIC|__GFP_NOWARN|__GFP_NOMEMALLOC);
>
> and I see in my logs
>
>      0.08%  call_site=ffffffff851d0cb0 ptr=0xffff8c04a4ca0000 bytes_req=10176 bytes_alloc=16384 gfp_flags=GFP_ATOMIC|__GFP_NOWARN|__GFP_NOMEMALLOC|__GFP_COMP
>
> That seems to me _more_ confusing because I would wonder "Where did that
> __GFP_COMP come from?"

Thank you for the comments. But I disagree.

When I use trace, I hope I can get the precise data rather than something
changed that I don't know , then I can get the correct conclusion or
direction on my issue.

Here my question is what the trace events are for if they don't provide the
real situation? I think that's not graceful and friendly.

>From my perspective, it'd be better to know my flags changed before checking
code lines one by one. In other words, I need a warning to reminder me on this,
then I can know quickly my process might do some incorrect things.

Regards,
Xiongwei