On 5/19/2021 4:29 AM, Joao Martins wrote:
On 5/18/21 8:56 PM, Jane Chu wrote:
On 5/18/2021 10:27 AM, Joao Martins wrote:
On 5/5/21 11:36 PM, Joao Martins wrote:
On 5/5/21 11:20 PM, Dan Williams wrote:
On Wed, May 5, 2021 at 12:50 PM Joao Martins <joao.m.martins@xxxxxxxxxx> wrote:
On 5/5/21 7:44 PM, Dan Williams wrote:
On Thu, Mar 25, 2021 at 4:10 PM Joao Martins <joao.m.martins@xxxxxxxxxx> wrote:
diff --git a/include/linux/memremap.h b/include/linux/memremap.h
index b46f63dcaed3..bb28d82dda5e 100644
--- a/include/linux/memremap.h
+++ b/include/linux/memremap.h
@@ -114,6 +114,7 @@ struct dev_pagemap {
struct completion done;
enum memory_type type;
unsigned int flags;
+ unsigned long align;
I think this wants some kernel-doc above to indicate that non-zero
means "use compound pages with tail-page dedup" and zero / PAGE_SIZE
means "use non-compound base pages".
[...]
The non-zero value must be
PAGE_SIZE, PMD_PAGE_SIZE or PUD_PAGE_SIZE.
Hmm, maybe it should be an
enum:
enum devmap_geometry {
DEVMAP_PTE,
DEVMAP_PMD,
DEVMAP_PUD,
}
I suppose a converter between devmap_geometry and page_size would be needed too? And maybe
the whole dax/nvdimm align values change meanwhile (as a followup improvement)?
I think it is ok for dax/nvdimm to continue to maintain their align
value because it should be ok to have 4MB align if the device really
wanted. However, when it goes to map that alignment with
memremap_pages() it can pick a mode. For example, it's already the
case that dax->align == 1GB is mapped with DEVMAP_PTE today, so
they're already separate concepts that can stay separate.
Gotcha.
I am reconsidering part of the above. In general, yes, the meaning of devmap @align
represents a slightly different variation of the device @align i.e. how the metadata is
laid out **but** regardless of what kind of page table entries we use vmemmap.
By using DEVMAP_PTE/PMD/PUD we might end up 1) duplicating what nvdimm/dax already
validates in terms of allowed device @align values (i.e. PAGE_SIZE, PMD_SIZE and PUD_SIZE)
2) the geometry of metadata is very much tied to the value we pick to @align at namespace
provisioning -- not the "align" we might use at mmap() perhaps that's what you referred
above? -- and 3) the value of geometry actually derives from dax device @align because we
will need to create compound pages representing a page size of @align value.
Using your example above: you're saying that dax->align == 1G is mapped with DEVMAP_PTEs,
in reality the vmemmap is populated with PMDs/PUDs page tables (depending on what archs
decide to do at vmemmap_populate()) and uses base pages as its metadata regardless of what
device @align. In reality what we want to convey in @geometry is not page table sizes, but
just the page size used for the vmemmap of the dax device. Additionally, limiting its
value might not be desirable... if tomorrow Linux for some arch supports dax/nvdimm
devices with 4M align or 64K align, the value of @geometry will have to reflect the 4M to
create compound pages of order 10 for the said vmemmap.
I am going to wait until you finish reviewing the remaining four patches of this series,
but maybe this is a simple misnomer (s/align/geometry/) with a comment but without
DEVMAP_{PTE,PMD,PUD} enum part? Or perhaps its own struct with a value and enum a
setter/getter to audit its value? Thoughts?
Good points there.
My understanding is that dax->align conveys granularity of size while
carving out a namespace it's a geometry attribute loosely akin to sector size of a spindle
disk. I tend to think that device pagesize has almost no relation to "align" in that, it's
possible to have 1G "align" and 4K pagesize, or verse versa. That is, with the advent of compound page
support, it is possible to totally separate the two concepts.
How about adding a new option to "ndctl create-namespace" that describes
device creator's desired pagesize, and another parameter to describe whether the pagesize shall
be fixed or allowed to be split up, such that, if the intention is to never split up 2M pagesize, then it
would be possible to save a lot metadata space on the device?
Maybe that can be selected by the driver too, but it's an interesting point you raise
should we settle with the geometry (e.g. like a geometry sysfs entry IIUC your
suggestion?). device-dax for example would use geometry == align and therefore save space
(like what I propose in patch 10). But fsdax would retain the default that is geometry =
PAGE_SIZE and align = PMD_SIZE should it want to split pages.
Let's see, I think this is what we have today
| align hpagesize geometry hpage-splittable
=======================================================
devdax | 4K..1G 2M,1G 4K artificially no
fsdax | 4K..1G 2M 4K yes
So a hard no-split means (hpagesize == geometry), and that does not apply
to fsdax for now. But is it not possible in future? Some customer prefers
an optional guarantee that their DAX hpage never been splitted up for
the sake of
rdma efficiency.
Interestingly, devmap poisoning always occur at @align level regardless of @geometry.
Yeah, it's a simplification that's not ideal, because after all,
error-blast-radius != UserMapping-pagesize.
What I am not sure is what value (vs added complexity) it brings to allow geometry *value*
to be selecteable by user given that so far we seem to only ever initialize metadata as
either sets of base pages [*] or sets of compound pages (of a size). And the difference
between both can possibly be summarized to split-ability like you say.
[*] that optionally can are morphed into compound pages by driver
Agreed. For this series, it's simpler not to make the
compound-page-size selectable.
thanks,
-jane
