linux-kernel - Re: copy on write for splice() from file to pipe?

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Message-ID: <Y+XLuYh+kC+4wTRi@casper.infradead.org>
Date:   Fri, 10 Feb 2023 04:44:41 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Stefan Metzmacher <metze@...ba.org>,
        Jens Axboe <axboe@...nel.dk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API Mailing List <linux-api@...r.kernel.org>,
        io-uring <io-uring@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Samba Technical <samba-technical@...ts.samba.org>
Subject: Re: copy on write for splice() from file to pipe?

On Fri, Feb 10, 2023 at 03:06:26PM +1100, Dave Chinner wrote:
> So while I was pondering the complexity of this and watching a great
> big shiny rocket create lots of heat, light and noise, it occurred

That was kind of fun

> to me that we already have a mechanism for preventing page cache
> data from being changed while the folios are under IO:
> SB_I_STABLE_WRITES and folio_wait_stable().

I thought about bringing that up, but it's not quite right.  That works
great for writeback, but it only works for writeback.  We'd need to track
another per-folio counter ... it'd be like the page pinning kerfuffle,
only worse.  And for such a rare thing it seems like a poor use of 32
bits of per-page state.  Not to mention that you can effectively block
all writes to a file for an indefinite time by splicing pages to a pipe
that you then never read from.