Re: [PATCH v2 6/8] mm: prevent get_user_pages() from overflowing page refcount

[Ajay Kaher]

ïOn 17/10/19, 9:58 PM, "Ajay Kaher" <akaher@xxxxxxxxxx> wrote:
    
> > This seems to have the same issue as the 4.9 stable version [1], in not
> > touching the arch-specific gup.c variants.
> >    
> > [1]
> > https://lore.kernel.org/lkml/6650323f-dbc9-f069-000b-f6b0f941a065@xxxxxxx/
>    
> Thanks Vlastimil for highlighting this here.
> 
> Yes, arch-specific gup.c variants also need to handle not only for 4.4.y,
> however it should be handled till 4.19.y. I believe it's better to start
> from 4.19.y and then backport those changes till 4.4.y.
>    
> Affected areas of gup.c (where page->count have been used) are:
> #1: get_page() used in these files and this is safe as
>        it's defined in mm.h (here it's already taken care of)
> #2: get_head_page_multiple() has following:
>               VM_BUG_ON_PAGE(page_count(page) == 0, page);
>          Need to change this to:
>               VM_BUG_ON_PAGE(page_ref_zero_or_close_to_overflow(page), page);
> #3: Some of the files have used page_cache_get_speculative(),
>        page_cache_add_speculative() with combination of compound_head(),
>        this scenario needs to be handled as it was handled here:
>            https://lore.kernel.org/stable/1570581863-12090-7-git-send-email-akaher@xxxxxxxxxx/
>    
> Please share with me any suggestions or patches if you have already  
> worked on this.
>    
> Could we handle arch-specific gup.c in different patch sets and 
> let these patches to merge to 4.4.y?
  
Vlastimil, please suggest if it's fine to merge these patches to 4.4.y
and handle arch-specific gup.c in different patch sets starts from 4.19.y,
then backport all the way to 4.4.y. 

Greg, any suggestion from your side.

>    - Ajay