Re: [RFC PATCH] block, fs: use FOLL_LONGTERM as gup_flags for direct IO

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Mar 6, 2025 at 6:07 PM Sooyong Suk <s.suk@samsung.com> wrote:
>
> > On Fri, Mar 7, 2025 at 12:26 AM Christoph Hellwig <hch@infradead.org>
> > wrote:
> > >
> > > On Thu, Mar 06, 2025 at 04:40:56PM +0900, Sooyong Suk wrote:
> > > > There are GUP references to pages that are serving as direct IO
> > buffers.
> > > > Those pages can be allocated from CMA pageblocks despite they can be
> > > > pinned until the DIO is completed.
> > >
> > > direct I/O is eactly the case that is not FOLL_LONGTERM and one of the
> > > reasons to even have the flag.  So big fat no to this.
> > >
> >
>
> Understood.
>
> > Hello, thank you for your comment.
> > We, Sooyong and I, wanted to get some opinions about this FOLL_LONGTERM
> > for direct I/O as CMA memory got pinned pages which had been pinned from
> > direct io.
> >
> > > You also completely failed to address the relevant mailinglist and
> > > maintainers.
> >
> > I added block maintainer Jens Axboe and the block layer maillinst here,
> > and added Suren and Sandeep, too.

I'm very far from being a block layer expert :)

>
> Then, what do you think of using PF_MEMALLOC_PIN for this context as below?
> This will only remove __GFP_MOVABLE from its allocation flag.
> Since __bio_iov_iter_get_pages() indicates that it will pin user or kernel pages,
> there seems to be no reason not to use this process flag.

I think this will help you only when the pages are faulted in but if
__get_user_pages() finds an already mapped page which happens to be
allocated from CMA, it will not migrate it. So, you might still end up
with unmovable pages inside CMA.

>
> block/bio.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/block/bio.c b/block/bio.c
> index 65c796ecb..671e28966 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -1248,6 +1248,7 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>         unsigned len, i = 0;
>         size_t offset;
>         int ret = 0;
> +       unsigned int flags;
>
>         /*
>          * Move page array up in the allocated memory for the bio vecs as far as
> @@ -1267,9 +1268,11 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>          * result to ensure the bio's total size is correct. The remainder of
>          * the iov data will be picked up in the next bio iteration.
>          */
> +       flags = memalloc_pin_save();
>         size = iov_iter_extract_pages(iter, &pages,
>                                       UINT_MAX - bio->bi_iter.bi_size,
>                                       nr_pages, extraction_flags, &offset);
> +       memalloc_pin_restore(flags);
>         if (unlikely(size <= 0))
>                 return size ? size : -EFAULT;
>
>