Re: [RFC PATCH v2 02/18] splice: don't steal page cache folios

[David Hildenbrand (Arm)]

On 11/4/26 14:00, Kiryl Shutsemau (Meta) wrote:
> From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> 
> PG_rcu_free holds back the reuse of a folio's memory until a grace
> period after the folio is freed.  A stolen page cache pipe buffer is
> reused without being freed: remove_mapping() takes the folio out of the
> page cache and the caller keeps it.  fuse_try_move_folio() puts it into
> a fuse file, where writes change it under a reader that found it before
> the steal.
> 
> Drop ->try_steal from page_cache_pipe_buf_ops.  fuse_try_move_folio()
> and virtio_console's pipe_to_sg() then copy, as they already do when
> stealing fails.  SPLICE_F_MOVE from the page cache into a fuse file or
> a virtio console now copies.
> 
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> [kas: rewrote the changelog]
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
> ---
>  fs/splice.c | 51 ---------------------------------------------------

Removing code is always nice, even independent of the other changes :)

The description doesn't really describe the impact of this change. I assume a
slowdown of certain workloads (does anybody really care?), and temporary
increased memory consumption (as we keep the pagecache page).

No idea how often stealing actually succeeds ... a bit of history why it was
added in the first place and why we no longer care now would be nice.

-- 
Cheers,

David