linux-kernel - Re: [PATCH 00/40] Memory allocation profiling

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Date:   Wed, 3 May 2023 12:24:44 +0200
From:   Petr Tesařík <petr@...arici.cz>
To:     Kent Overstreet <kent.overstreet@...ux.dev>
Cc:     Michal Hocko <mhocko@...e.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
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Subject: Re: [PATCH 00/40] Memory allocation profiling

On Wed, 3 May 2023 05:54:43 -0400
Kent Overstreet <kent.overstreet@...ux.dev> wrote:

> On Wed, May 03, 2023 at 11:50:51AM +0200, Petr Tesařík wrote:
> > On Wed, 3 May 2023 09:51:49 +0200
> > Michal Hocko <mhocko@...e.com> wrote:
> >   
> > > On Wed 03-05-23 03:34:21, Kent Overstreet wrote:
> > >[...]  
> > > > We've made this as clean and simple as posssible: a single new macro
> > > > invocation per allocation function, no calling convention changes (that
> > > > would indeed have been a lot of churn!)    
> > > 
> > > That doesn't really make the concern any less relevant. I believe you
> > > and Suren have made a great effort to reduce the churn as much as
> > > possible but looking at the diffstat the code changes are clearly there
> > > and you have to convince the rest of the community that this maintenance
> > > overhead is really worth it.  
> > 
> > I believe this is the crucial point.
> > 
> > I have my own concerns about the use of preprocessor macros, which goes
> > against the basic idea of a code tagging framework (patch 13/40).
> > AFAICS the CODE_TAG_INIT macro must be expanded on the same source code
> > line as the tagged code, which makes it hard to use without further
> > macros (unless you want to make the source code unreadable beyond
> > imagination). That's why all allocation functions must be converted to
> > macros.
> > 
> > If anyone ever wants to use this code tagging framework for something
> > else, they will also have to convert relevant functions to macros,
> > slowly changing the kernel to a minefield where local identifiers,
> > struct, union and enum tags, field names and labels must avoid name
> > conflict with a tagged function. For now, I have to remember that
> > alloc_pages is forbidden, but the list may grow.  
> 
> No, we've got other code tagging applications (that have already been
> posted!) and they don't "convert functions to macros" in the way this
> patchset does - they do introduce new macros, but as new identifiers,
> which we do all the time.

Yes, new all-lowercase macros which do not expand to a single
identifier are still added under include/linux. It's unfortunate IMO,
but it's a fact of life. You have a point here.

> This was simply the least churny way to hook memory allocations.

This is a bold statement. You certainly know what you plan to do, but
other people keep coming up with ideas... Like, anyone would like to
tag semaphore use: up() and down()?

Don't get me wrong. I can see how the benefits of code tagging, and I
agree that my concerns are not very strong. I just want that the
consequences are understood and accepted, and they don't take us by
surprise.

Petr T