linux-kernel - Re: [PATCH v14 02/17] splice: Add a func to do a splice from a buffered file without ITER

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Date:   Sat, 18 Feb 2023 09:25:07 +0000
From:   David Howells <dhowells@...hat.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     dhowells@...hat.com, Jens Axboe <axboe@...nel.dk>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Hillf Danton <hdanton@...a.com>, linux-fsdevel@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH v14 02/17] splice: Add a func to do a splice from a buffered file without ITER_PIPE

Ming Lei <ming.lei@...hat.com> wrote:

> > +	/* Work out how much data we can actually add into the pipe */
> > +	used = pipe_occupancy(pipe->head, pipe->tail);
> > +	npages = max_t(ssize_t, pipe->max_usage - used, 0);
> > +	len = min_t(size_t, len, npages * PAGE_SIZE);
> 
> Do we need to consider offset in 1st page here?

Well, it won't break since we check further on that we don't overrun the ring,
but it's probably a bit more efficient to subtract the offset into the page at
this point.

That said, we don't know how big the first folio is yet, though I'm not sure
if that matters.

David