Paolo Abeni [off-list ref] wrote:

IMHO this function uses a bit too much labels and would be more easy to
read, e.g. moving the above chunk of code in conditional branch.

Maybe.  I was trying to put the fast path up at the top without the slow path
bits in it, but I can put the "insufficient_space" bit there.

Even without such change, I think the above 'goto try_again;'
introduces an unneeded conditional, as at this point we know 'fragsz <=
fsize'.

Good point.

quoted

+		cache->pfmemalloc = folio_is_pfmemalloc(spare);
+		if (cache->folio)
+			goto reload;

I think there is some problem with the above.

If cache->folio is != NULL, and cache->folio was not pfmemalloc-ed
while the spare one is, it looks like the wrong policy will be used.
And should be even worse if folio was pfmemalloc-ed while spare is not.

I think moving 'cache->pfmemalloc' initialization...

quoted

+	}
+

... here should fix the above.

Yeah.  We might have raced with someone else or been moved to another cpu and
there might now be a folio we can allocate from.

quoted

+	/* Reset page count bias and offset to start of new frag */
+	cache->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
+	offset = folio_size(folio);
+	goto try_again;

What if fragsz > PAGE_SIZE, we are consistently unable to allocate an
high order page, but order-0, pfmemalloc-ed page allocation is
successful? It looks like this could become an unbounded loop?

It shouldn't.  It should go:

	try_again:
		if (fragsz > offset)
			goto insufficient_space;
	insufficient_space:
		/* See if we can refurbish the current folio. */
		...
		fsize = folio_size(folio);
		if (unlikely(fragsz > fsize))
			goto frag_too_big;
	frag_too_big:
		...
		return NULL;

Though for safety's sake, it would make sense to put in a size check in the
case we fail to allocate a larger-order folio.

quoted

 		do {
 			struct page *page = pages[i++];
 			size_t part = min_t(size_t, PAGE_SIZE - off, len);
-
-			ret = -EIO;
-			if (WARN_ON_ONCE(!sendpage_ok(page)))
+			bool put = false;
+
+			if (PageSlab(page)) {

I'm a bit concerned from the above. If I read correctly, tcp 0-copy

Well, splice()-to-tcp will; MSG_ZEROCOPY is unaffected.

will go through that for every page, even if the expected use-case is
always !PageSlub(page). compound_head() could be costly if the head
page is not hot on cache and I'm not sure if that could be the case for
tcp 0-copy. The bottom line is that I fear a possible regression here.

I can put the PageSlab() check inside the sendpage_ok() so the page flag is
only checked once.  But PageSlab() doesn't check the headpage, only the page
it is given.  sendpage_ok() is more the problem as it also calls
page_count().  I could drop the check.

David