Hi,

On Tue, Jan 30, 2024 at 10:22:11AM +0530, Anshuman Khandual wrote:

On 1/29/24 17:21, Alexandru Elisei wrote:

quoted

Hi,

On Mon, Jan 29, 2024 at 02:54:20PM +0530, Anshuman Khandual wrote:

quoted

On 1/25/24 22:12, Alexandru Elisei wrote:

quoted

The CMA_ALLOC_SUCCESS, respectively CMA_ALLOC_FAIL, are increased by one
after each cma_alloc() function call. This is done even though cma_alloc()
can allocate an arbitrary number of CMA pages. When looking at
/proc/vmstat, the number of successful (or failed) cma_alloc() calls
doesn't tell much with regards to how many CMA pages were allocated via
cma_alloc() versus via the page allocator (regular allocation request or
PCP lists refill).

This can also be rather confusing to a user who isn't familiar with the
code, since the unit of measurement for nr_free_cma is the number of pages,
but cma_alloc_success and cma_alloc_fail count the number of cma_alloc()
function calls.

Let's make this consistent, and arguably more useful, by having
CMA_ALLOC_SUCCESS count the number of successfully allocated CMA pages, and
CMA_ALLOC_FAIL count the number of pages the cma_alloc() failed to
allocate.

For users that wish to track the number of cma_alloc() calls, there are
tracepoints for that already implemented.

Signed-off-by: Alexandru Elisei <redacted>
---
 mm/cma.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/cma.c b/mm/cma.c
index f49c95f8ee37..dbf7fe8cb1bd 100644
--- a/mm/cma.c
+++ b/mm/cma.c

@@ -517,10 +517,10 @@ struct page *cma_alloc(struct cma *cma, unsigned long count,
 	pr_debug("%s(): returned %p\n", __func__, page);
 out:
 	if (page) {
-		count_vm_event(CMA_ALLOC_SUCCESS);
+		count_vm_events(CMA_ALLOC_SUCCESS, count);
 		cma_sysfs_account_success_pages(cma, count);
 	} else {
-		count_vm_event(CMA_ALLOC_FAIL);
+		count_vm_events(CMA_ALLOC_FAIL, count);
 		if (cma)
 			cma_sysfs_account_fail_pages(cma, count);
 	}

Without getting into the merits of this patch - which is actually trying to do
semantics change to /proc/vmstat, wondering how is this even related to this
particular series ? If required this could be debated on it's on separately.

Having the number of CMA pages allocated and the number of CMA pages freed
allows someone to infer how many tagged pages are in use at a given time:

That should not be done in CMA which is a generic multi purpose allocator.

Ah, ok. Let me rephrase that: Having the number of CMA pages allocated, the
number of failed CMA page allocations and the number of freed CMA pages
allows someone to infer how many CMA pages are in use at a given time.
That's valuable information for software designers and system
administrators, as it allows them to tune the number of CMA pages available
in a system.

Or put another way: what would you consider to be more useful?  Knowing the
number of cma_alloc()/cma_release() calls, or knowing the number of pages
that cma_alloc()/cma_release() allocated or freed?

quoted

(allocated CMA pages - CMA pages allocated by drivers* - CMA pages
released) * 32. That is valuable information for software and hardware
designers.

Besides that, for every iteration of the series, this has proven invaluable
for discovering bugs with freeing and/or reserving tag storage pages.

I am afraid that might not be enough justification for getting something
merged mainline.

quoted

*that would require userspace reading cma_alloc_success and
cma_release_success before any tagged allocations are performed.

While assuming that no other non-memory-tagged CMA based allocation amd free
call happens in the meantime ? That would be on real thin ice.

I suppose arm64 tagged memory specific allocation or free related counters
need to be created on the caller side, including arch_free_pages_prepare().

I'll think about this. At the very least, I can add tracepoints.

Thanks,
Alex