Re: [PATCH v2 1/2] mm/madvise: introduce PR_MADV_SELF flag to process_madvise()

[Pedro Falcato <pedro.falcato@gmail.com>]

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