On 21.10.24 18:23, Lorenzo Stoakes wrote:
On Mon, Oct 21, 2024 at 06:00:20PM +0200, David Hildenbrand wrote:
[snip]
To summarise for on-list:
* MADV_FREE, while ostensibly being a 'lazy free' mechanism, has the
ability to be 'cancelled' if you write to the memory. Also, after the
freeing is complete, you can write to the memory to reuse it, the mapping
is still there.
* For hardware poison markers it makes sense to drop them as you're
effectively saying 'I am done with this range that is now unbacked and
expect to get an empty page should I use it now'. UFFD WP I am not sure
about but presumably also fine.
* However, guard pages are different - if you 'cancel' and you are left
with a block of memory allocated to you by a pthread or userland
allocator implementation, you don't want to then no longer be protected
from overrunning into other thread memory.
Agreed. What happens on MADV_DONTNEED/MADV_FREE on guard pages? Ignored or
error? It sounds like a usage "error" to me (in contrast to munmap()).
It's ignored, no errror. On MADV_DONTNEED we already left the guard pages in
place, from v3 we will do the same for MADV_FREE.
I'm not sure I'd say it's an error per se, as somebody might have a use case
where they want to zap over a range but keep guard pages, perhaps an allocator
or something?
Hm, not sure I see use for that.
Staring at madvise_walk_vmas(), we return ENOMEM on VMA holes, but would
process PROT_NONE. So current behavior is at least consistent with
PROT_NONE handling (where something could be mapped, though).
No strong opinion.
Also the existing logic is that existing markers (HW poison, uffd-simulated HW
poison, uffd wp marker) are retained and no error raised on MADV_DONTNEED, and
no error on MADV_FREE either, so it'd be consistent with existing behaviour.
HW poison / uffd-simulated HW poison are expected to be zapped: it's
just like a mapped page with HWPOISON. So that is correct.
UFFD-WP behavior is ... weird. Would not expect MADV_DONTNEED to zap
uffd-wp entries.
Also semantically you are achieving what the calls expect you are freeing the
ranges since the guard page regions are unbacked so are already freed... so yeah
I don't think an error really makes sense here.
I you compare it to a VMA hole, it make sense to fail. If we treat it
like PROT_NONE, it make sense to skip them.
We might also be limiting use cases by assuming they might _only_ be used for
allocators and such.
I don't buy that as an argument, sorry :)
"Let's map the kernel writable into all user space because otherwise we
might be limiting use cases"
:P
--
Cheers,
David / dhildenb
