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

[David Hildenbrand <david@redhat.com>]

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.

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

-- 
Cheers,

David / dhildenb