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Date: Tue, 7 Nov 2023 08:08:07 -0700
From: Keith Busch <kbusch@kernel.org>
To: Kanchan Joshi <joshi.k@samsung.com>
Cc: Keith Busch <kbusch@meta.com>, linux-block@vger.kernel.org,
	linux-nvme@lists.infradead.org, io-uring@vger.kernel.org,
	axboe@kernel.dk, hch@lst.de, martin.petersen@oracle.com
Subject: Re: [PATCHv2 1/4] block: bio-integrity: directly map user buffers
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On Tue, Nov 07, 2023 at 03:55:14PM +0530, Kanchan Joshi wrote:
> On 11/6/2023 8:32 PM, Keith Busch wrote:
> > On Mon, Nov 06, 2023 at 11:18:03AM +0530, Kanchan Joshi wrote:
> >> On 10/27/2023 11:49 PM, Keith Busch wrote:
> >>> +	for (i = 0; i < nr_vecs; i = j) {
> >>> +		size_t size = min_t(size_t, bytes, PAGE_SIZE - offs);
> >>> +		struct folio *folio = page_folio(pages[i]);
> >>> +
> >>> +		bytes -= size;
> >>> +		for (j = i + 1; j < nr_vecs; j++) {
> >>> +			size_t next = min_t(size_t, PAGE_SIZE, bytes);
> >>> +
> >>> +			if (page_folio(pages[j]) != folio ||
> >>> +			    pages[j] != pages[j - 1] + 1)
> >>> +				break;
> >>> +			unpin_user_page(pages[j]);
> >>
> >> Is this unpin correct here?
> > 
> > Should be. The pages are bound to the folio, so this doesn't really
> > unpin the user page. It just drops a reference, and the folio holds the
> > final reference to the contiguous pages, which is released on
> > completion. 
> 
> But the completion is still going to see multiple pages and not one 
> (folio). The bip_for_each_vec loop is going to drop the reference again.
> I suspect it is not folio-aware.

The completion unpins once per bvec, not individual pages. The setup
creates multipage bvecs with only one pin remaining per bvec for all of
the bvec's pages. If a page can't be merged into the current bvec, then
that page is not unpinned and becomes the first page of to the next
bvec.