Re: [PATCH 1/3] mm: introduce VM_MAYBE_GUARD and make visible for guard regions

[Vlastimil Babka <vbabka@suse.cz>]

On 10/30/25 20:47, Lorenzo Stoakes wrote:
> On Thu, Oct 30, 2025 at 07:47:34PM +0100, Vlastimil Babka wrote:
>> >
>> > Could we use MADVISE_VMA_READ_LOCK mode (would be actually an improvement
>> > over the current MADVISE_MMAP_READ_LOCK), together with the atomic flag
>> > setting? I think the places that could race with us to cause RMW use vma
>> > write lock so that would be excluded. Fork AFAICS unfortunately doesn't (for
>> > the oldmm) and it probably would't make sense to start doing it. Maybe we
>> > could think of something to deal with this special case...
>>
>> During discussion with Pedro off-list I realized fork takes mmap lock for
>> write on the old mm, so if we kept taking mmap sem for read, then vma lock
>> for read in addition (which should be cheap enough, also we'd only need it
>> in case VM_MAYBE_GUARD is not yet set), and set the flag atomicaly, perhaps
>> that would cover all non-bening races?
>>
>>
> 
> We take VMA write lock in dup_mmap() on each mpnt (old VMA).

Ah yes I thought it was the new one.

> We take the VMA write lock (vma_start_write()) for each mpnt.
> 
> We then vm_area_dup() the mpnt to the new VMA before calling:
> 
> copy_page_range()
> -> vma_needs_copy()
> 
> Which is where the check is done.
> 
> So we are holding the VMA write lock, so a VMA read lock should suffice no?

Yeah, even better!

> For belts + braces we could atomically read the flag in vma_needs_copy(),
> though note it's intended VM_COPY_ON_FORK could have more than one flag.
> 
> We could drop that for now and be explicit.

Great!