On Wed, Jun 17, 2026 at 4:37 PM Oleg Nesterov <[email protected]> wrote: > > On 06/17, Mateusz Guzik wrote: > > > > There are trivial touch ups which can be done by adding a bunch of > > predicts and inlining kill_fasync if someone can be bothered. > > I was thinking about another change, see below. It assumes that in the > likely case another writer won't steal the pages from ->tmp_page[] > before we take pipe->mutex. > > I'm not sure this makes sense, and I have no idea how it would impact > performance in "real" workloads. > > Oleg. > --- > > diff --git a/fs/pipe.c b/fs/pipe.c > index 429b0714ec57..9f07f469830a 100644 > --- a/fs/pipe.c > +++ b/fs/pipe.c > @@ -131,7 +131,8 @@ struct anon_pipe_prealloc { > * pipe->mutex hold-time being shrunk. Any shortfall is covered by the > * in-lock alloc_page() fallback in anon_pipe_get_page(). > */ > -static void anon_pipe_get_page_prealloc(struct anon_pipe_prealloc *prealloc, > +static void anon_pipe_get_page_prealloc(struct pipe_inode_info *pipe, > + struct anon_pipe_prealloc *prealloc, > size_t total_len) > { > unsigned int want, i; > @@ -144,6 +145,11 @@ static void anon_pipe_get_page_prealloc(struct > anon_pipe_prealloc *prealloc, > want = min_t(unsigned int, DIV_ROUND_UP(total_len, PAGE_SIZE), > PIPE_PREALLOC_MAX); > > + for (i = 0; i < ARRAY_SIZE(pipe->tmp_page); i++) { > + if (pipe->tmp_page[i] && !--want) > + return; > + } > + > for (i = 0; i < want; i++) { > page = alloc_page(GFP_HIGHUSER | __GFP_ACCOUNT); > if (!page) > @@ -548,7 +554,7 @@ anon_pipe_write(struct kiocb *iocb, struct iov_iter *from) > if (unlikely(total_len == 0)) > return 0; > > - anon_pipe_get_page_prealloc(&prealloc, total_len); > + anon_pipe_get_page_prealloc(pipe, &prealloc, total_len); > > mutex_lock(&pipe->mutex); > >
As proposed this will guarantee a big write which fits fine into pages cached into tmp_page followed by a small write will have to resort to an allocation under the mutex, partially defeating the original patch. So you would need to add some provisions to check if you need to allocate something even in that case.
