From: Joao Martins <joao.m.martins@oracle.com>
To: Jason Gunthorpe <jgg@nvidia.com>,
Dan Williams <dan.j.williams@intel.com>
Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
Christoph Hellwig <hch@lst.de>,
Heiko Carstens <hca@linux.ibm.com>,
Vasily Gorbik <gor@linux.ibm.com>,
Christian Borntraeger <borntraeger@de.ibm.com>,
Linux NVDIMM <nvdimm@lists.linux.dev>,
linux-s390 <linux-s390@vger.kernel.org>,
Matthew Wilcox <willy@infradead.org>,
Alex Sierra <alex.sierra@amd.com>,
"Kuehling, Felix" <Felix.Kuehling@amd.com>,
Linux MM <linux-mm@kvack.org>,
Ralph Campbell <rcampbell@nvidia.com>,
Alistair Popple <apopple@nvidia.com>,
Vishal Verma <vishal.l.verma@intel.com>,
Dave Jiang <dave.jiang@intel.com>
Subject: Re: can we finally kill off CONFIG_FS_DAX_LIMITED
Date: Fri, 15 Oct 2021 01:22:41 +0100 [thread overview]
Message-ID: <5ca908e3-b4ad-dfef-d75f-75073d4165f7@oracle.com> (raw)
In-Reply-To: <20211014230439.GA3592864@nvidia.com>
On 10/15/21 00:04, Jason Gunthorpe wrote:
> 2) Denying FOLL_LONGTERM
> Once GUP has grabbed the page we can call is_zone_device_page() on
> the struct page. If true we can check page->pgmap and read some
> DENY_FOLL_LONGTERM flag from there
>
I had proposed something similar to that:
https://lore.kernel.org/linux-mm/6a18179e-65f7-367d-89a9-d5162f10fef0@oracle.com/
Albeit I was using pgmap->type and was relying on get_dev_pagemap() ref
as opposed to after grabbing the page. I can ressurect that with some
adjustments to use pgmap flags to check DENY_LONGTERM flag (and set it
on fsdax[*]) and move the check to after try_grab_page(). That is provided
the other alternative with special page bit isn't an option anymore.
[*] which begs the question on whether fsdax is the *only* that needs the flag?
> 3) Different refcounts for pud/pmd pages
>
> Ideally DAX cases would not do this (ie Joao is fixing device-dax)
> but in the interm we can just loop over the PUD/PMD in all
> cases. Looping is safe for THP AFAIK. I described how this can work
> here:
>
> https://lore.kernel.org/all/20211013174140.GJ2744544@nvidia.com/
>
> After that there are only two remaining uses:
>
> 4) The pud/pmd_devmap() in vm_normal_page() should just go
> away. ZONE_DEVICE memory with struct pages SHOULD be a normal
> page. This also means dropping pte_special too.
>
> 5) dev_pagemap_mapping_shift() - I don't know what this does
> but why not use the is_zone_device_page() approach from 2?
>
dev_pagemap_mapping_shift() does a lookup to figure out
which order is the page table entry represents. is_zone_device_page()
is already used to gate usage of dev_pagemap_mapping_shift(). I think
this might be an artifact of the same issue as 3) in which PMDs/PUDs
are represented with base pages and hence you can't do what the rest
of the world does with:
tk->size_shift = page_shift(compound_head(p));
... as page_shift() would just return PAGE_SHIFT (as compound_order() is 0).
next prev parent reply other threads:[~2021-10-15 0:23 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20210820054340.GA28560@lst.de>
[not found] ` <20210823160546.0bf243bf@thinkpad>
[not found] ` <20210823214708.77979b3f@thinkpad>
[not found] ` <CAPcyv4jijqrb1O5OOTd5ftQ2Q-5SVwNRM7XMQ+N3MAFxEfvxpA@mail.gmail.com>
[not found] ` <e250feab-1873-c91d-5ea9-39ac6ef26458@oracle.com>
[not found] ` <CAPcyv4jYXPWmT2EzroTa7RDz1Z68Qz8Uj4MeheQHPbBXdfS4pA@mail.gmail.com>
[not found] ` <20210824202449.19d524b5@thinkpad>
[not found] ` <CAPcyv4iFeVDVPn6uc=aKsyUvkiu3-fK-N16iJVZQ3N8oT00hWA@mail.gmail.com>
2021-10-14 23:04 ` can we finally kill off CONFIG_FS_DAX_LIMITED Jason Gunthorpe
2021-10-15 0:22 ` Joao Martins [this message]
2021-10-18 23:30 ` Jason Gunthorpe
2021-10-19 4:26 ` Dan Williams
2021-10-19 14:20 ` Jason Gunthorpe
2021-10-19 15:20 ` Joao Martins
2021-10-19 15:38 ` Felix Kuehling
2021-10-19 17:38 ` Dan Williams
2021-10-19 17:54 ` Jason Gunthorpe
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