Re: [PATCH v4 bpf-next 06/10] lib/buildid: implement sleepable build_id_parse() API

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Aug 8, 2024 at 11:40 AM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> On Wed, Aug 07, 2024 at 04:40:25PM GMT, Andrii Nakryiko wrote:
> > Extend freader with a flag specifying whether it's OK to cause page
> > fault to fetch file data that is not already physically present in
> > memory. With this, it's now easy to wait for data if the caller is
> > running in sleepable (faultable) context.
> >
> > We utilize read_cache_folio() to bring the desired folio into page
> > cache, after which the rest of the logic works just the same at folio level.
> >
> > Suggested-by: Omar Sandoval <osandov@fb.com>
> > Cc: Shakeel Butt <shakeel.butt@linux.dev>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> > ---
> >  lib/buildid.c | 44 ++++++++++++++++++++++++++++----------------
> >  1 file changed, 28 insertions(+), 16 deletions(-)
> >
> > diff --git a/lib/buildid.c b/lib/buildid.c
> > index 5e6f842f56f0..e1c01b23efd8 100644
> > --- a/lib/buildid.c
> > +++ b/lib/buildid.c
> > @@ -20,6 +20,7 @@ struct freader {
> >                       struct folio *folio;
> >                       void *addr;
> >                       loff_t folio_off;
> > +                     bool may_fault;
> >               };
> >               struct {
> >                       const char *data;
> > @@ -29,12 +30,13 @@ struct freader {
> >  };
> >
> >  static void freader_init_from_file(struct freader *r, void *buf, u32 buf_sz,
> > -                                struct address_space *mapping)
> > +                                struct address_space *mapping, bool may_fault)
> >  {
> >       memset(r, 0, sizeof(*r));
> >       r->buf = buf;
> >       r->buf_sz = buf_sz;
> >       r->mapping = mapping;
> > +     r->may_fault = may_fault;
> >  }
> >
> >  static void freader_init_from_mem(struct freader *r, const char *data, u64 data_sz)
> > @@ -63,6 +65,11 @@ static int freader_get_folio(struct freader *r, loff_t file_off)
> >       freader_put_folio(r);
> >
> >       r->folio = filemap_get_folio(r->mapping, file_off >> PAGE_SHIFT);
> > +
> > +     /* if sleeping is allowed, wait for the page, if necessary */
> > +     if (r->may_fault && (IS_ERR(r->folio) || !folio_test_uptodate(r->folio)))
> > +             r->folio = read_cache_folio(r->mapping, file_off >> PAGE_SHIFT, NULL, NULL);
>
> Willy's network fs comment is bugging me. If we pass NULL for filler,
> the kernel will going to use fs's read_folio() callback. I have checked
> read_folio() for fuse and nfs and it seems like for at least these two
> filesystems the callback is accessing file->private_data. So, if the elf
> file is on these filesystems, we might see null accesses.
>

Isn't that just a huge problem with the read_cache_folio() interface
then? That file is optional, in general, but for some specific FS
types it's not. How generic code is supposed to know this?

Or maybe it's a bug with the nfs_read_folio() and fuse_read_folio()
implementation that they can't handle NULL file argument?
netfs_read_folio(), for example, seems to be working with file == NULL
just fine.

Matthew, can you please advise what's the right approach here? I can,
of course, always get file refcount, but most of the time it will be
just an unnecessary overhead, so ideally I'd like to avoid that. But
if I have to check each read_folio callback implementation to know
whether it's required or not, then that's not great...