linux-kernel - Re: [PATCH v2 1/2] mm/madvise: introduce PR_MADV_SELF flag to process

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Message-ID: <u64scsk52b3ek4b7fh72tdylkf3qh537txcqhvozmaasrlug3r@eqsmstvs324c>
Date: Tue, 24 Sep 2024 13:51:11 +0100
From: Pedro Falcato <pedro.falcato@...il.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Vlastimil Babka <vbabka@...e.cz>, "Liam R . Howlett" <Liam.Howlett@...cle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Arnd Bergmann <arnd@...db.de>, 
	Shakeel Butt <shakeel.butt@...ux.dev>, linux-api@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Minchan Kim <minchan@...nel.org>, 
	Richard Henderson <richard.henderson@...aro.org>, Ivan Kokshaysky <ink@...assic.park.msu.ru>, 
	Matt Turner <mattst88@...il.com>, linux-alpha@...r.kernel.org, 
	Thomas Bogendoerfer <tsbogend@...ha.franken.de>, linux-mips@...r.kernel.org, 
	"James E . J . Bottomley" <James.Bottomley@...senpartnership.com>, Helge Deller <deller@....de>, linux-parisc@...r.kernel.org, 
	Chris Zankel <chris@...kel.net>, Max Filippov <jcmvbkbc@...il.com>
Subject: Re: [PATCH v2 1/2] mm/madvise: introduce PR_MADV_SELF flag to
 process_madvise()

On Tue, Sep 24, 2024 at 12:16:27PM GMT, Lorenzo Stoakes wrote:
> process_madvise() was conceived as a useful means for performing a vector
> of madvise() operations on a remote process's address space.
> 
> However it's useful to be able to do so on the current process also. It is
> currently rather clunky to do this (requiring a pidfd to be opened for the
> current process) and introduces unnecessary overhead in incrementing
> reference counts for the task and mm.
> 
> Avoid all of this by providing a PR_MADV_SELF flag, which causes
> process_madvise() to simply ignore the pidfd parameter and instead apply
> the operation to the current process.
> 

How about simply defining a pseudo-fd PIDFD_SELF in the negative int space?
There's precedent for it in the fs space (AT_FDCWD). I think it's more ergonomic
and if you take out the errno space we have around 2^31 - 4096 available sentinel
values.

e.g:

/* AT_FDCWD = -10, -1 is dangerous, pick a different value */
#define PIDFD_SELF   -11

int pidfd = target_pid == getpid() ? PIDFD_SELF : pidfd_open(...);
process_madvise(pidfd, ...);


What do you think?

-- 
Pedro