Re: [PATCH v2 4/8] mm/gup: track FOLL_PIN pages

Kirill A. Shutemov
2020-01-30T11:31:26Z

On Wed, Jan 29, 2020 at 10:44:50PM -0800, John Hubbard wrote:
> On 1/29/20 5:51 AM, Kirill A. Shutemov wrote:
> > > +/**
> > > + * page_dma_pinned() - report if a page is pinned for DMA.
> > > + *
> > > + * This function checks if a page has been pinned via a call to
> > > + * pin_user_pages*().
> > > + *
> > > + * For non-huge pages, the return value is partially fuzzy: false is not fuzzy,
> > > + * because it means "definitely not pinned for DMA", but true means "probably
> > > + * pinned for DMA, but possibly a false positive due to having at least
> > > + * GUP_PIN_COUNTING_BIAS worth of normal page references".
> > > + *
> > > + * False positives are OK, because: a) it's unlikely for a page to get that many
> > > + * refcounts, and b) all the callers of this routine are expected to be able to
> > > + * deal gracefully with a false positive.
> >
> > I wounder if we should reverse the logic and name -- page_not_dma_pinned()
> > or something -- too emphasise that we can only know for sure when the page
> > is not pinned, but not necessary when it is.
> >
>
> This is an interesting point. I agree that it's worth maybe adding information
> into the function name, but I'd like to keep the bool "positive", because there
> will be a number of callers that ask "if it is possibly dma-pinned, then ...".
> So combining that, how about this function name:
>
> page_maybe_dma_pinned()
>
> , which I could live with and I think would be acceptable?

I would still prefer the negative version, but up to you.

> > I see opportunity to split the patch further.
>
>
> ah, OK. I wasn't sure how far to go before I get tagged for "excessive
> patch splitting"! haha. Anyway, are you suggesting to put the
> page_ref_sub_return() routine into it's own patch?
>
> Another thing to split out would be adding the flags to the remaining
> functions, such as undo_dev_pagemap(). That burns quite a few lines of
> diff. Anything else to split out?

Nothing I see immediately.

>
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index 0a55dec68925..b1079aaa6f24 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -958,6 +958,11 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
> > > */
> > > WARN_ONCE(flags & FOLL_COW, "mm: In follow_devmap_pmd with FOLL_COW set");
> > > + /* FOLL_GET and FOLL_PIN are mutually exclusive. */
> > > + if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
> > > + (FOLL_PIN | FOLL_GET)))
> >
> > Too many parentheses.
>
>
> OK, I'll remove at least one. :)

I see two.

--
Kirill A. Shutemov