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.
