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SubjectRe: [patch rfc] towards supporting O_NONBLOCK on regular files
==> Regarding Re: [patch rfc] towards supporting O_NONBLOCK on regular files; "Stephen C. Tweedie" <sct@redhat.com> adds:

sct> Hi, On Mon, 2004-10-11 at 19:58, Jeff Moyer wrote:

sct> I think it's worth getting this right in the long term, though.
sct> Getting readahead of indirect blocks right has other benefits too ---
sct> eg. we may be able to fix the situation where we end up trying to read
sct> indirect blocks before we've even submitted the IO for the previous
sct> data blocks, breaking the IO pipeline ordering.
>> So for the short term, are you an advocate of the patch posted?

sct> In the short term, can't we just disable readahead for O_NONBLOCK?
sct> That has true non-blocking semantics --- if the data is already
sct> available we return it, but if not, it's up to somebody else to
sct> retrieve it.

sct> That's exactly what you want if you're genuinely trying to avoid
sct> blocking at all costs on a really hot event loop, and the semantics
sct> seem to make sense to me. It's not that different from the networking
sct> case where no amount of read() on a non-blocking fd will get you more
sct> data unless there's another process somewhere filling the stream.

Yes, that sounds like a fine idea. Here is a patch which does this.
Andrew, I know you only want bug fixes, but I'd like to get this into your
queue for post 2.6.9, if possible.

Thanks,

Jeff

--- linux-2.6.9-rc4-mm1/mm/filemap.c~ 2004-10-12 12:40:01.000000000 -0400
+++ linux-2.6.9-rc4-mm1/mm/filemap.c 2004-10-12 12:53:13.202396656 -0400
@@ -692,7 +692,7 @@ void do_generic_mapping_read(struct addr
unsigned long index, end_index, offset;
loff_t isize;
struct page *cached_page;
- int error;
+ int error, nonblock = filp->f_flags & O_NONBLOCK;
struct file_ra_state ra = *_ra;

cached_page = NULL;
@@ -721,16 +721,27 @@ void do_generic_mapping_read(struct addr
nr = nr - offset;

cond_resched();
- page_cache_readahead(mapping, &ra, filp, index);
+ if (!nonblock)
+ page_cache_readahead(mapping, &ra, filp, index);

find_page:
page = find_get_page(mapping, index);
if (unlikely(page == NULL)) {
+ if (nonblock) {
+ desc->error = -EWOULDBLOCK;
+ break;
+ }
handle_ra_miss(mapping, &ra, index);
goto no_cached_page;
}
- if (!PageUptodate(page))
+ if (!PageUptodate(page)) {
+ if (nonblock) {
+ page_cache_release(page);
+ desc->error = -EWOULDBLOCK;
+ break;
+ }
goto page_not_up_to_date;
+ }
page_ok:

/* If users can be writing to this page using arbitrary
@@ -976,7 +987,7 @@ __generic_file_aio_read(struct kiocb *io
desc.error = 0;
do_generic_file_read(filp,ppos,&desc,file_read_actor);
retval += desc.written;
- if (!retval) {
+ if (!retval || desc.error) {
retval = desc.error;
break;
}
-
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