On Thu, Aug 8, 2024 at 2:02 PM Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote:
>
> On Thu, Aug 08, 2024 at 01:15:52PM GMT, Andrii Nakryiko wrote:
> > On Thu, Aug 8, 2024 at 11:40 AM Shakeel Butt <shakeel.butt@xxxxxxxxx> 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@xxxxxx>
> > > > Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> > > > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > > > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > > > ---
> > > > 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.
>
> If you go a bit down in netfs_alloc_request() there is the following
> code:
>
> if (rreq->netfs_ops->init_request) {
> ret = rreq->netfs_ops->init_request(rreq, file);
> ...
> ...
>
> I think this init_request is pointing to nfs_netfs_init_request which
> calls nfs_file_open_context(file) and access filp->private_data.
That's "nfs", which we know requires a file. For netfs implementations
(cifs_init_request() and v9fs_init_request()), they both treat file as
optional consistently.
But regardless, that's just pointless code archeology, I'll just pass
the file reference unconditionally.
>
> >
> > 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...
>
> I don't think we will need file refcnt. We have mmap lock in read mode
> in this context because we are accessing vma and this vma has reference
> to the file. So, this file can not go away under us here.
Yep, good point, then it's not a problem, thanks! Will update.
