Re: + mm-add-sys-kernel-slab-cache-cache_dma32.patch added to -mm tree

Re: + mm-add-sys-kernel-slab-cache-cache_dma32.patch added to -mm tree

[Michal Hocko]

On Tue 19-03-19 11:37:51, Andrew Morton wrote:
> From: Nicolas Boichat <drinkcat@chromium.org>
> Subject: mm: add /sys/kernel/slab/cache/cache_dma32
> 
> A previous patch in this series adds support for SLAB_CACHE_DMA32 kmem
> caches.  This adds the corresponding /sys/kernel/slab/cache/cache_dma32
> entries, and fixes slabinfo tool.

I believe I have asked and didn't get a satisfactory answer before IIRC. Who
is going to consume this information?
-- 
Michal Hocko
SUSE Labs

Re: + mm-add-sys-kernel-slab-cache-cache_dma32.patch added to -mm tree

[Nicolas Boichat]

On Wed, Mar 20, 2019 at 3:18 AM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Tue 19-03-19 11:37:51, Andrew Morton wrote:
> > From: Nicolas Boichat <drinkcat@chromium.org>
> > Subject: mm: add /sys/kernel/slab/cache/cache_dma32
> >
> > A previous patch in this series adds support for SLAB_CACHE_DMA32 kmem
> > caches.  This adds the corresponding /sys/kernel/slab/cache/cache_dma32
> > entries, and fixes slabinfo tool.
>
> I believe I have asked and didn't get a satisfactory answer before IIRC. Who
> is going to consume this information?

No answer from me, but as a reminder, I added this note on the patch
(https://patchwork.kernel.org/patch/10720491/):
"""
There were different opinions on whether this sysfs entry should
be added, so I'll leave it up to the mm/slub maintainers to decide
whether they want to pick this up, or drop it.
"""

> --
> Michal Hocko
> SUSE Labs

Re: + mm-add-sys-kernel-slab-cache-cache_dma32.patch added to -mm tree

[Christopher Lameter]

On Tue, 19 Mar 2019, Michal Hocko wrote:

>
> I believe I have asked and didn't get a satisfactory answer before IIRC. Who
> is going to consume this information?

The slabinfo tool consumes this information.

Re: + mm-add-sys-kernel-slab-cache-cache_dma32.patch added to -mm tree

[Michal Hocko]

On Wed 20-03-19 00:37:33, Cristopher Lameter wrote:
> On Tue, 19 Mar 2019, Michal Hocko wrote:
> 
> >
> > I believe I have asked and didn't get a satisfactory answer before IIRC. Who
> > is going to consume this information?
> 
> The slabinfo tool consumes this information.

Slabinfo just prints that information without any additional logic
AFAICS. So this doesn't look like a usecase to add an API to maintain
for ever.

Or is anybody using slabinfo to use this information for anything
useful?

-- 
Michal Hocko
SUSE Labs

Re: + mm-add-sys-kernel-slab-cache-cache_dma32.patch added to -mm tree

[Michal Hocko]

On Wed 20-03-19 08:17:52, Nicolas Boichat wrote:
> On Wed, Mar 20, 2019 at 3:18 AM Michal Hocko <mhocko@kernel.org> wrote:
> >
> > On Tue 19-03-19 11:37:51, Andrew Morton wrote:
> > > From: Nicolas Boichat <drinkcat@chromium.org>
> > > Subject: mm: add /sys/kernel/slab/cache/cache_dma32
> > >
> > > A previous patch in this series adds support for SLAB_CACHE_DMA32 kmem
> > > caches.  This adds the corresponding /sys/kernel/slab/cache/cache_dma32
> > > entries, and fixes slabinfo tool.
> >
> > I believe I have asked and didn't get a satisfactory answer before IIRC. Who
> > is going to consume this information?
> 
> No answer from me, but as a reminder, I added this note on the patch
> (https://patchwork.kernel.org/patch/10720491/):
> """
> There were different opinions on whether this sysfs entry should
> be added, so I'll leave it up to the mm/slub maintainers to decide
> whether they want to pick this up, or drop it.
> """

We have a terrible track rescord of exporting data to userspace that
kick back much later. So I am really convinced that adding new user
visible data should be justified by a useful usecase. Exporting just
because we can is a terrible justification if you ask me.
-- 
Michal Hocko
SUSE Labs

Re: + mm-add-sys-kernel-slab-cache-cache_dma32.patch added to -mm tree

[Andrew Morton]

This patch is presently in limbo.   Should we just drop it?


From: Nicolas Boichat <drinkcat@chromium.org>
Subject: mm: add /sys/kernel/slab/cache/cache_dma32

The patch "mm: add support for kmem caches in DMA32 zone" added support
for SLAB_CACHE_DMA32 kmem caches.  This patch adds the corresponding
/sys/kernel/slab/cache/cache_dma32 entries, and updates the slabinfo tool.

Link: http://lkml.kernel.org/r/20181210011504.122604-4-drinkcat@chromium.org
Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Hsin-Yi Wang <hsinyi@chromium.org>
Cc: Huaisheng Ye <yehs1@lenovo.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Sasha Levin <Alexander.Levin@microsoft.com>
Cc: Tomasz Figa <tfiga@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Yingjoe Chen <yingjoe.chen@mediatek.com>
Cc: Yong Wu <yong.wu@mediatek.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 Documentation/ABI/testing/sysfs-kernel-slab |    9 +++++++++
 mm/slub.c                                   |   11 +++++++++++
 tools/vm/slabinfo.c                         |    7 ++++++-
 3 files changed, 26 insertions(+), 1 deletion(-)

--- a/Documentation/ABI/testing/sysfs-kernel-slab~mm-add-sys-kernel-slab-cache-cache_dma32
+++ a/Documentation/ABI/testing/sysfs-kernel-slab
@@ -106,6 +106,15 @@ Description:
 		are from ZONE_DMA.
 		Available when CONFIG_ZONE_DMA is enabled.
 
+What:		/sys/kernel/slab/cache/cache_dma32
+Date:		December 2018
+KernelVersion:	4.21
+Contact:	Nicolas Boichat <drinkcat@chromium.org>
+Description:
+		The cache_dma32 file is read-only and specifies whether objects
+		are from ZONE_DMA32.
+		Available when CONFIG_ZONE_DMA32 is enabled.
+
 What:		/sys/kernel/slab/cache/cpu_slabs
 Date:		May 2007
 KernelVersion:	2.6.22
--- a/mm/slub.c~mm-add-sys-kernel-slab-cache-cache_dma32
+++ a/mm/slub.c
@@ -5112,6 +5112,14 @@ static ssize_t cache_dma_show(struct kme
 SLAB_ATTR_RO(cache_dma);
 #endif
 
+#ifdef CONFIG_ZONE_DMA32
+static ssize_t cache_dma32_show(struct kmem_cache *s, char *buf)
+{
+	return sprintf(buf, "%d\n", !!(s->flags & SLAB_CACHE_DMA32));
+}
+SLAB_ATTR_RO(cache_dma32);
+#endif
+
 static ssize_t usersize_show(struct kmem_cache *s, char *buf)
 {
 	return sprintf(buf, "%u\n", s->usersize);
@@ -5452,6 +5460,9 @@ static struct attribute *slab_attrs[] =
 #ifdef CONFIG_ZONE_DMA
 	&cache_dma_attr.attr,
 #endif
+#ifdef CONFIG_ZONE_DMA32
+	&cache_dma32_attr.attr,
+#endif
 #ifdef CONFIG_NUMA
 	&remote_node_defrag_ratio_attr.attr,
 #endif
--- a/tools/vm/slabinfo.c~mm-add-sys-kernel-slab-cache-cache_dma32
+++ a/tools/vm/slabinfo.c
@@ -29,7 +29,7 @@ struct slabinfo {
 	char *name;
 	int alias;
 	int refs;
-	int aliases, align, cache_dma, cpu_slabs, destroy_by_rcu;
+	int aliases, align, cache_dma, cache_dma32, cpu_slabs, destroy_by_rcu;
 	unsigned int hwcache_align, object_size, objs_per_slab;
 	unsigned int sanity_checks, slab_size, store_user, trace;
 	int order, poison, reclaim_account, red_zone;
@@ -534,6 +534,8 @@ static void report(struct slabinfo *s)
 		printf("** Hardware cacheline aligned\n");
 	if (s->cache_dma)
 		printf("** Memory is allocated in a special DMA zone\n");
+	if (s->cache_dma32)
+		printf("** Memory is allocated in a special DMA32 zone\n");
 	if (s->destroy_by_rcu)
 		printf("** Slabs are destroyed via RCU\n");
 	if (s->reclaim_account)
@@ -602,6 +604,8 @@ static void slabcache(struct slabinfo *s
 		*p++ = '*';
 	if (s->cache_dma)
 		*p++ = 'd';
+	if (s->cache_dma32)
+		*p++ = 'D';
 	if (s->hwcache_align)
 		*p++ = 'A';
 	if (s->poison)
@@ -1208,6 +1212,7 @@ static void read_slab_dir(void)
 			slab->aliases = get_obj("aliases");
 			slab->align = get_obj("align");
 			slab->cache_dma = get_obj("cache_dma");
+			slab->cache_dma32 = get_obj("cache_dma32");
 			slab->cpu_slabs = get_obj("cpu_slabs");
 			slab->destroy_by_rcu = get_obj("destroy_by_rcu");
 			slab->hwcache_align = get_obj("hwcache_align");
_

Re: + mm-add-sys-kernel-slab-cache-cache_dma32.patch added to -mm tree

[Michal Hocko]

On Thu 25-04-19 21:46:15, Andrew Morton wrote:
> This patch is presently in limbo.   Should we just drop it?

There was no strong justification for it except "we alredy do export dma
cache so why not dma32". Christopher was arguing that slabinfo (in tree
tool) is going to use it but that merely prints it without any
additional considerations. So I would just drop it until there is a real
use case (aka somebody is going to use it for something _useful_).
-- 
Michal Hocko
SUSE Labs