Re: [RFC PATCH 0/5] add process_madvise() flags to modify behaviour

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Tue, May 20, 2025 at 05:28:35PM +0200, David Hildenbrand wrote:
> On 19.05.25 22:52, Lorenzo Stoakes wrote:
> > REVIEWERS NOTES:
> > ================
> >
> > This is a VERY EARLY version of the idea, it's relatively untested, and I'm
> > 'putting it out there' for feedback. Any serious version of this will add a
> > bunch of self-tests to assert correct behaviour and I will more carefully
> > confirm everything's working.
> >
> > This is based on discussion arising from Usama's series [0], SJ's input on
> > the thread around process_madvise() behaviour [1] (and a subsequent
> > response by me [2]) and prior discussion about a new madvise() interface
> > [3].
> >
> > [0]: https://lore.kernel.org/linux-mm/20250515133519.2779639-1-usamaarif642@gmail.com/
> > [1]: https://lore.kernel.org/linux-mm/20250517162048.36347-1-sj@kernel.org/
> > [2]: https://lore.kernel.org/linux-mm/e3ba284c-3cb1-42c1-a0ba-9c59374d0541@lucifer.local/
> > [3]: https://lore.kernel.org/linux-mm/c390dd7e-0770-4d29-bb0e-f410ff6678e3@lucifer.local/
> >
> > ================
> >
> > Currently, we are rather restricted in how madvise() operations
> > proceed. While effort has been put in to expanding what process_madvise()
> > can do (that is - unrestricted application of advice to the local process
> > alongside recent improvements on the efficiency of TLB operations over
> > these batvches), we are still constrained by existing madvise() limitations
> > and default behaviours.
> >
> > This series makes use of the currently unused flags field in
> > process_madvise() to provide more flexiblity.
> >
>
> In general, sounds like an interesting approach.

Thanks!

If you agree this is workable, then I'll go ahead and put some more effort
into writing tests etc. on the next respin.

>
> > It introduces four flags:
> >
> > 1. PMADV_SKIP_ERRORS
> >
> > Currently, when an error arises applying advice in any individual VMA
> > (keeping in mind that a range specified to madvise() or as part of the
> > iovec passed to process_madvise()), the operation stops where it is and
> > returns an error.
> >
> > This might not be the desired behaviour of the user, who may wish instead
> > for the operation to be 'best effort'. By setting this flag, that behaviour
> > is obtained.
> >
> > Since process_madvise() would trivially, if skipping errors, simply return
> > the input vector size, we instead return the number of entries in the
> > vector which completed successfully without error.
>
> I would focus only on adding flags that we absolutely need to make the use
> case we have in mind work. We can always add other flags as we see fit for
> them (IOW, when really required ;) ).
>
> Regarding MADV_HUGEPAGE / MADV_NOHUGEPAGE, this would not be required,
> right?

Sure, we can restrict this to only supported flags to be conservative.

The idea was though that somebody might want to simply do a 'best effort'
change.

However at the same time it's possibly a wee bit dangerous...

>
> >
> > The PMADV_SKIP_ERRORS flag implicitly implies PMADV_NO_ERROR_ON_UNMAPPED.
> >
> > 2. PMADV_NO_ERROR_ON_UNMAPPED
> >
> > Currently madvise() has the peculiar behaviour of, if the range specified
> > to it contains unmapped range(s), completing the full operation, but
> > ultimately returning -ENOMEM.
> >
> > In the case of process_madvise(), this is fatal, as the operation will stop
> > immediately upon this occurring.
> >
> > By setting PMADV_NO_ERROR_ON_UNMAPPED, the user can indicate that it wishes
> > unmapped areas to simply be entirely ignored.
>
> Again, is this really required? Couldn't we glue that to
> PMADV_ENTIRE_ADDRESS_SPACE for our use case? After all, I don't expect
> anybody to have something mapped into *the entire address space*.

Well, I think it's an ongoing issue that unmapped entries cause the whole thing
to break, I do think it makes sense to make this _generally_ available, actually.

Obviously we should probably make PMADV_ENTIRE_MAPPING imply
PMADV_NO_ERROR_ON_UNMAPPED in the same way that PMADV_SKIP_ERRORS implies
PMADV_NO_ERROR_ON_UNMAPPED.

And yes I don't think any sane person would map the entirety of the 64-bit
address space :P

>
> Well, okay, maybe on 32bit, but still ... :)

32 what? :P I deny its existence... (ugh ok I guess I have to ack it, but
even in that case it's not very likely either :)

>
> >
> > 3. PMADV_SET_FORK_EXEC_DEFAULT
> >
> > It may be desirable for a user to specify that all VMAs mapped in a process
> > address space default to having an madvise() behaviour established by
> > default, in such a fashion as that this persists across fork/exec.
>
> This is very specific for MADV_HUGEPAGE only, so I wonder how we could
> either avoid that flag or just make it clear that it shall stick around ...
>
> Having that sad, PMADV_SET_FORK_EXEC_DEFAULT is rather a suboptimal name :(

Yeah it's horrid, see Pedro's suggestions, e.g. PMADV_SET_DEFAULT |
PMADV_INHERIT_EXEC.

I even wonder about PMADV_, but that's probably fine. PRMADV sucks, PADV
sort of loses the mm bit, PMADV is probably best we can do!

>
> --
> Cheers,
>
> David / dhildenb
>