Re: [PATCH 2/5] mm/mseal: move madvise() logic to mm/madvise.c

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

On Mon, Jul 14, 2025 at 05:41:45PM +0200, David Hildenbrand wrote:
> On 14.07.25 17:31, Pedro Falcato wrote:
> > On Mon, Jul 14, 2025 at 05:03:03PM +0200, David Hildenbrand wrote:
> > > [...]
> > >
> > > But now I wonder, why is it okay to discard anon pages in a MAP_PRIVATE file
> > > mapping?
> >
> > IIRC this was originally suggested by Linus, on one of the versions introducing
> > mseal. But the gist is that discarding pages is okay if you could already zero
> > them manually, using e.g memset. Hence the writeability checks.
>
> What you can do is
>
> a) mmap(PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, ...)
>
> b) modify content (write, whatever)
>
> c) mprotect(PROT_READ)
>
> d) mseal()
>
> But then still do
>
> madvise(MADV_DONTNEED)
>
> to discard.
>
>
> There is no writability anymore.

Well, you can mprotect() writable it again :)

>
> (Just a note that, with hugetlb, it is fairly common to mmap(MAP_PRIVATE)
> empty files and only work with anonymous pages.)

This just makes me think that we should be checking VM_MAYWRITE anyway which
squares this circle.

Otherwise you can do 'workarounds' with mprotect() generally.

It's really only meaningful for a MAP_PRIVATE file-backed mapping of a read-only
file.

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