Re: [PATCH] mm: remove extra ZONE_DEVICE struct page refcount
From: Ralph Campbell
Date: Mon Sep 14 2020 - 19:53:36 EST
On 9/14/20 4:10 PM, Dan Williams wrote:
On Mon, Sep 14, 2020 at 3:45 PM Ralph Campbell <rcampbell@xxxxxxxxxx> wrote:
ZONE_DEVICE struct pages have an extra reference count that complicates the
code for put_page() and several places in the kernel that need to check the
reference count to see that a page is not being used (gup, compaction,
migration, etc.). Clean up the code so the reference count doesn't need to
be treated specially for ZONE_DEVICE.
Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
---
Matthew Wilcox, Ira Weiny, and others have complained that ZONE_DEVICE
struct page reference counting is ugly/broken. This is my attempt to
fix it and it works for the HMM migration self tests.
Can you link to a technical description of what's broken? Or better
yet, summarize that argument in the changelog?
I'm only sending this out as a RFC since I'm not that familiar with the
DAX, PMEM, XEN, and other uses of ZONE_DEVICE struct pages allocated
with devm_memremap_pages() or memremap_pages() but my best reading of
the code looks like it might be OK. I could use help testing these
configurations.
Back in the 4.15 days I could not convince myself that some code paths
blindly assumed that pages with refcount==0 were on an lru list. Since
then, struct page has been reorganized to not collide the ->pgmap back
pointer with the ->lru list and there have been other cleanups for
page pinning that might make this incremental cleanup viable.
You also need to fix up ext4_break_layouts() and
xfs_break_dax_layouts() to expect ->_refcount is 0 instead of 1. This
also needs some fstests exposure.
Got it. Thanks!
I have a modified THP migration patch series that applies on top of
this one and is cleaner since I don't have to add code to handle the
+1 reference count. The link below is for the earlier v2:
("mm/hmm/nouveau: add THP migration to migrate_vma_*")
https://lore.kernel.org/linux-mm/20200902165830.5367-1-rcampbell@xxxxxxxxxx
arch/powerpc/kvm/book3s_hv_uvmem.c | 1 -
drivers/gpu/drm/nouveau/nouveau_dmem.c | 1 -
include/linux/memremap.h | 6 +--
include/linux/mm.h | 39 ---------------
lib/test_hmm.c | 1 -
mm/gup.c | 44 -----------------
mm/memremap.c | 20 ++++----
mm/migrate.c | 5 --
mm/swap.c | 66 +++++++++++---------------
9 files changed, 41 insertions(+), 142 deletions(-)
This diffstat is indeed appealing.
diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
index 84e5a2dc8be5..00d97050d7ff 100644
--- a/arch/powerpc/kvm/book3s_hv_uvmem.c
+++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
@@ -711,7 +711,6 @@ static struct page *kvmppc_uvmem_get_page(unsigned long gpa, struct kvm *kvm)
dpage = pfn_to_page(uvmem_pfn);
dpage->zone_device_data = pvt;
- get_page(dpage);
lock_page(dpage);
return dpage;
out_clear:
diff --git a/drivers/gpu/drm/nouveau/nouveau_dmem.c b/drivers/gpu/drm/nouveau/nouveau_dmem.c
index a13c6215bba8..2a4bbe01a455 100644
--- a/drivers/gpu/drm/nouveau/nouveau_dmem.c
+++ b/drivers/gpu/drm/nouveau/nouveau_dmem.c
@@ -324,7 +324,6 @@ nouveau_dmem_page_alloc_locked(struct nouveau_drm *drm)
return NULL;
}
- get_page(page);
lock_page(page);
return page;
}
diff --git a/include/linux/memremap.h b/include/linux/memremap.h
index 4e9c738f4b31..7dd9802d2612 100644
--- a/include/linux/memremap.h
+++ b/include/linux/memremap.h
@@ -67,9 +67,9 @@ enum memory_type {
struct dev_pagemap_ops {
/*
- * Called once the page refcount reaches 1. (ZONE_DEVICE pages never
- * reach 0 refcount unless there is a refcount bug. This allows the
- * device driver to implement its own memory management.)
+ * Called once the page refcount reaches 0. The reference count is
+ * reset to 1 before calling page_free(). This allows the
+ * device driver to implement its own memory management.
I'd clarify the order events / responsibility of the common core
page_free() and the device specific page_free(). At the same time, why
not update drivers to expect that the page is already refcount==0 on
entry? Seems odd to go through all this trouble to make the reference
count appear to be zero to the wider kernel but expect that drivers
get a fake reference on entry to their ->page_free() callbacks.
Good point.
Since set_page_refcounted() is defined in mm_interal.h I would have to
move the definition to someplace like page_ref.h or have the drivers
cal init_page_count() or set_page_count() since get_page() calls
VM_BUG_ON_PAGE() if refcount == 0.
I'll move set_page_refcounted() since that is what the page allocator
uses and seems named for the purpose.