linux-kernel - Re: [PATCH 3/5] mm: Rework return value for copy_one

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Date:   Tue, 22 Sep 2020 11:29:29 -0400
From:   Peter Xu <peterx@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Jason Gunthorpe <jgg@...pe.ca>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Michal Hocko <mhocko@...e.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Christoph Hellwig <hch@....de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 3/5] mm: Rework return value for copy_one_pte()

On Tue, Sep 22, 2020 at 12:11:29AM -0700, John Hubbard wrote:
> On 9/21/20 2:17 PM, Peter Xu wrote:
> > There's one special path for copy_one_pte() with swap entries, in which
> > add_swap_count_continuation(GFP_ATOMIC) might fail.  In that case we'll return
> 
> I might be looking at the wrong place, but the existing code seems to call
> add_swap_count_continuation(GFP_KERNEL), not with GFP_ATOMIC?

Ah, I wanted to reference the one in swap_duplicate().

> 
> > the swp_entry_t so that the caller will release the locks and redo the same
> > thing with GFP_KERNEL.
> > 
> > It's confusing when copy_one_pte() must return a swp_entry_t (even if all the
> > ptes are non-swap entries).  More importantly, we face other requirement to
> > extend this "we need to do something else, but without the locks held" case.
> > 
> > Rework the return value into something easier to understand, as defined in enum
> > copy_mm_ret.  We'll pass the swp_entry_t back using the newly introduced union
> 
> I like the documentation here, but it doesn't match what you did in the patch.
> Actually, the documentation had the right idea (enum, rather than #define, for
> COPY_MM_* items). Below...

Yeah actually my very initial version has it as an enum, then I changed it to
macros because I started to want it return negative as errors.  However funnily
in the current version copy_one_pte() won't return an error anymore... So
probably, yes, it should be a good idea to get the enum back.

Also we should be able to drop the negative handling too with copy_ret, though
it should be in the next patch.

> 
> > copy_mm_data parameter.
> > 
> > Another trivial change is to move the reset of the "progress" counter into the
> > retry path, so that we'll reset it for other reasons too.
> > 
> > This should prepare us with adding new return codes, very soon.
> > 
> > Signed-off-by: Peter Xu <peterx@...hat.com>
> > ---
> >   mm/memory.c | 42 +++++++++++++++++++++++++++++-------------
> >   1 file changed, 29 insertions(+), 13 deletions(-)
> > 
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 7525147908c4..1530bb1070f4 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -689,16 +689,24 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
> >   }
> >   #endif
> > +#define  COPY_MM_DONE               0
> > +#define  COPY_MM_SWAP_CONT          1
> 
> Those should be enums, so as to get a little type safety and other goodness from
> using non-macro items.
> 
> ...
> > @@ -866,13 +877,18 @@ static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> >   	pte_unmap_unlock(orig_dst_pte, dst_ptl);
> >   	cond_resched();
> > -	if (entry.val) {
> > -		if (add_swap_count_continuation(entry, GFP_KERNEL) < 0)
> > +	switch (copy_ret) {
> > +	case COPY_MM_SWAP_CONT:
> > +		if (add_swap_count_continuation(data.entry, GFP_KERNEL) < 0)
> >   			return -ENOMEM;
> > -		progress = 0;
> 
> Yes. Definitely a little cleaner to reset this above, instead of here.
> 
> > +		break;
> > +	default:
> > +		break;
> 
> I assume this no-op noise is to placate the compiler and/or static checkers. :)

This is (so far) for COPY_MM_DONE.  I normally will cover all cases in a
"switch()" and here "default" is for it.  Even if I covered all the
possibilities, I may still tend to keep one "default" and a WARN_ON_ONCE(1) to
make sure nothing I've missed.  Not sure whether that's the ideal way, though.

Thanks,

-- 
Peter Xu