On Friday, November 25, 2016 4:51:03 PM CET Vitaly Wool wrote:

On Fri, Nov 25, 2016 at 9:41 AM, Arnd Bergmann [off-list ref] wrote:

quoted

On Friday, November 25, 2016 8:38:25 AM CET Vitaly Wool wrote:

quoted

quoted

quoted

diff --git a/mm/z3fold.c b/mm/z3fold.c
index e282ba073e77..66ac7a7dc934 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c

@@ -884,7 +884,7 @@ static int __init init_z3fold(void)
 {
      /* Fail the initialization if z3fold header won't fit in one chunk */
      if (sizeof(struct z3fold_header) > ZHDR_SIZE_ALIGNED) {
-             pr_err("z3fold: z3fold_header size (%d) is bigger than "
+             pr_err("z3fold: z3fold_header size (%zd) is bigger than "
                      "the chunk size (%d), can't proceed\n",
                      sizeof(struct z3fold_header) , ZHDR_SIZE_ALIGNED);
              return -E2BIG;

The embedded "z3fold: " prefix here should be removed
as there's a pr_fmt that also adds it.

The test looks like it should be a BUILD_BUG_ON rather
than any runtime test too.

It used to be BUILD_BUG_ON but we deliberately changed that because
sizeof(spinlock_t) gets bloated in debug builds, so it just won't
build with default CHUNK_SIZE.

Could this be improved by making the CHUNK_SIZE bigger depending on
the debug options?

I don't see how silently enforcing a suboptimal configuration is
better than failing the initialization (so that you can adjust
CHUNK_SIZE yourself). I can add something descriptive to
Documentation/vm/z3fold.txt for that matter.

Failing at runtime when you know it's broken at compile-time
seems wrong, too. If you can't use z3fold with spinlock debugging,
you may as well hide the option in Kconfig based on the other ones.

Printing a runtime warning for the suboptimal configuration but
making it work anyway is probably better than just failing.

quoted

Alternatively, how about using a bit_spin_lock instead of raw_spin_lock?
That would guarantee a fixed size for the lock and make z3fold_header
always 24 bytes (on 32-bit architectures) or 40 bytes
(on 64-bit architectures). You could even play some tricks with the
first_num field to make it fit in the same word as the lock and make the
structure fit into 32 bytes if you care about that.

That is interesting. Actually I can have that bit in page->private and
then I don't need to handle headless pages in a special way, that
sounds appealing. However, there is a warning about bit_spin_lock
performance penalty. Do you know how big it is?

No idea, sorry. On x86, test_and_set_bit() seems to be only
one instruction to test/set the bit, followed by a conditional
branch, compared to a cmpxchg() for the raw_spin_lock(), so the
fast path seems pretty much the same.

	Arnd

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