| Message ID | 20190208075649.3025-1-jhubbard@nvidia.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | mm: put_user_page() call site conversion first | expand |
On Thu, Feb 07, 2019 at 11:56:47PM -0800, john.hubbard@gmail.com wrote: > From: John Hubbard <jhubbard@nvidia.com> > > Hi, > > It seems about time to post these initial patches: I think we have pretty > good consensus on the concept and details of the put_user_pages() approach. > Therefore, here are the first two patches, to get started on converting the > get_user_pages() call sites to use put_user_page(), instead of put_page(). > This is in order to implement tracking of get_user_page() pages. > > A discussion of the overall problem is below. > > As mentioned in patch 0001, the steps are to fix the problem are: > > 1) Provide put_user_page*() routines, intended to be used > for releasing pages that were pinned via get_user_pages*(). > > 2) Convert all of the call sites for get_user_pages*(), to > invoke put_user_page*(), instead of put_page(). This involves dozens of > call sites, and will take some time. > > 3) After (2) is complete, use get_user_pages*() and put_user_page*() to > implement tracking of these pages. This tracking will be separate from > the existing struct page refcounting. > > 4) Use the tracking and identification of these pages, to implement > special handling (especially in writeback paths) when the pages are > backed by a filesystem. > > This write up is lifted from the RFC v2 patchset cover letter [1]: > > Overview > ======== > > Some kernel components (file systems, device drivers) need to access > memory that is specified via process virtual address. For a long time, the > API to achieve that was get_user_pages ("GUP") and its variations. However, > GUP has critical limitations that have been overlooked; in particular, GUP > does not interact correctly with filesystems in all situations. That means > that file-backed memory + GUP is a recipe for potential problems, some of > which have already occurred in the field. > > GUP was first introduced for Direct IO (O_DIRECT), allowing filesystem code > to get the struct page behind a virtual address and to let storage hardware > perform a direct copy to or from that page. This is a short-lived access > pattern, and as such, the window for a concurrent writeback of GUP'd page > was small enough that there were not (we think) any reported problems. > Also, userspace was expected to understand and accept that Direct IO was > not synchronized with memory-mapped access to that data, nor with any > process address space changes such as munmap(), mremap(), etc. > > Over the years, more GUP uses have appeared (virtualization, device > drivers, RDMA) that can keep the pages they get via GUP for a long period > of time (seconds, minutes, hours, days, ...). This long-term pinning makes > an underlying design problem more obvious. > > In fact, there are a number of key problems inherent to GUP: > > Interactions with file systems > ============================== > > File systems expect to be able to write back data, both to reclaim pages, > and for data integrity. Allowing other hardware (NICs, GPUs, etc) to gain > write access to the file memory pages means that such hardware can dirty > the pages, without the filesystem being aware. This can, in some cases > (depending on filesystem, filesystem options, block device, block device > options, and other variables), lead to data corruption, and also to kernel > bugs of the form: > > kernel BUG at /build/linux-fQ94TU/linux-4.4.0/fs/ext4/inode.c:1899! > backtrace: > ext4_writepage > __writepage > write_cache_pages > ext4_writepages > do_writepages > __writeback_single_inode > writeback_sb_inodes > __writeback_inodes_wb > wb_writeback > wb_workfn > process_one_work > worker_thread > kthread > ret_from_fork > > ...which is due to the file system asserting that there are still buffer > heads attached: > > ({ \ > BUG_ON(!PagePrivate(page)); \ > ((struct buffer_head *)page_private(page)); \ > }) > > Dave Chinner's description of this is very clear: > > "The fundamental issue is that ->page_mkwrite must be called on every > write access to a clean file backed page, not just the first one. > How long the GUP reference lasts is irrelevant, if the page is clean > and you need to dirty it, you must call ->page_mkwrite before it is > marked writeable and dirtied. Every. Time." > > This is just one symptom of the larger design problem: filesystems do not > actually support get_user_pages() being called on their pages, and letting > hardware write directly to those pages--even though that pattern has been > going on since about 2005 or so. > > Long term GUP > ============= > > Long term GUP is an issue when FOLL_WRITE is specified to GUP (so, a > writeable mapping is created), and the pages are file-backed. That can lead > to filesystem corruption. What happens is that when a file-backed page is > being written back, it is first mapped read-only in all of the CPU page > tables; the file system then assumes that nobody can write to the page, and > that the page content is therefore stable. Unfortunately, the GUP callers > generally do not monitor changes to the CPU pages tables; they instead > assume that the following pattern is safe (it's not): > > get_user_pages() > > Hardware can keep a reference to those pages for a very long time, > and write to it at any time. Because "hardware" here means "devices > that are not a CPU", this activity occurs without any interaction > with the kernel's file system code. > > for each page > set_page_dirty > put_page() > > In fact, the GUP documentation even recommends that pattern. > > Anyway, the file system assumes that the page is stable (nothing is writing > to the page), and that is a problem: stable page content is necessary for > many filesystem actions during writeback, such as checksum, encryption, > RAID striping, etc. Furthermore, filesystem features like COW (copy on > write) or snapshot also rely on being able to use a new page for as memory > for that memory range inside the file. > > Corruption during write back is clearly possible here. To solve that, one > idea is to identify pages that have active GUP, so that we can use a bounce > page to write stable data to the filesystem. The filesystem would work > on the bounce page, while any of the active GUP might write to the > original page. This would avoid the stable page violation problem, but note > that it is only part of the overall solution, because other problems > remain. > > Other filesystem features that need to replace the page with a new one can > be inhibited for pages that are GUP-pinned. This will, however, alter and > limit some of those filesystem features. The only fix for that would be to > require GUP users to monitor and respond to CPU page table updates. > Subsystems such as ODP and HMM do this, for example. This aspect of the > problem is still under discussion. > > Direct IO > ========= > > Direct IO can cause corruption, if userspace does Direct-IO that writes to > a range of virtual addresses that are mmap'd to a file. The pages written > to are file-backed pages that can be under write back, while the Direct IO > is taking place. Here, Direct IO races with a write back: it calls > GUP before page_mkclean() has replaced the CPU pte with a read-only entry. > The race window is pretty small, which is probably why years have gone by > before we noticed this problem: Direct IO is generally very quick, and > tends to finish up before the filesystem gets around to do anything with > the page contents. However, it's still a real problem. The solution is > to never let GUP return pages that are under write back, but instead, > force GUP to take a write fault on those pages. That way, GUP will > properly synchronize with the active write back. This does not change the > required GUP behavior, it just avoids that race. > > > [1] https://lkml.kernel.org/r/20190204052135.25784-1-jhubbard@nvidia.com > > Cc: Christian Benvenuti <benve@cisco.com> > Cc: Christoph Hellwig <hch@infradead.org> > Cc: Christopher Lameter <cl@linux.com> > Cc: Dan Williams <dan.j.williams@intel.com> > Cc: Dave Chinner <david@fromorbit.com> > Cc: Dennis Dalessandro <dennis.dalessandro@intel.com> > Cc: Doug Ledford <dledford@redhat.com> > Cc: Jan Kara <jack@suse.cz> > Cc: Jason Gunthorpe <jgg@ziepe.ca> > Cc: Jérôme Glisse <jglisse@redhat.com> > Cc: Matthew Wilcox <willy@infradead.org> > Cc: Michal Hocko <mhocko@kernel.org> > Cc: Mike Rapoport <rppt@linux.ibm.com> > Cc: Mike Marciniszyn <mike.marciniszyn@intel.com> > Cc: Ralph Campbell <rcampbell@nvidia.com> > Cc: Tom Talpey <tom@talpey.com> > > John Hubbard (2): > mm: introduce put_user_page*(), placeholder versions > infiniband/mm: convert put_page() to put_user_page*() A bit late but, FWIW: Reviewed-by: Ira Weiny <ira.weiny@intel.com> John these are the pages sitting in your gup_dma/first_steps branch here, correct? https://github.com/johnhubbard/linux.git > > drivers/infiniband/core/umem.c | 7 +- > drivers/infiniband/core/umem_odp.c | 2 +- > drivers/infiniband/hw/hfi1/user_pages.c | 11 +-- > drivers/infiniband/hw/mthca/mthca_memfree.c | 6 +- > drivers/infiniband/hw/qib/qib_user_pages.c | 11 +-- > drivers/infiniband/hw/qib/qib_user_sdma.c | 6 +- > drivers/infiniband/hw/usnic/usnic_uiom.c | 7 +- > include/linux/mm.h | 24 ++++++ > mm/swap.c | 82 +++++++++++++++++++++ > 9 files changed, 129 insertions(+), 27 deletions(-) > > -- > 2.20.1 >
On 2/14/19 4:23 PM, Ira Weiny wrote: > On Thu, Feb 07, 2019 at 11:56:47PM -0800, john.hubbard@gmail.com wrote: >> From: John Hubbard <jhubbard@nvidia.com> [...] >> mm: introduce put_user_page*(), placeholder versions >> infiniband/mm: convert put_page() to put_user_page*() > > A bit late but, FWIW: > > Reviewed-by: Ira Weiny <ira.weiny@intel.com> > > John these are the pages sitting in your gup_dma/first_steps branch here, > correct? > > https://github.com/johnhubbard/linux.git > That's an old branch. In fact, just deleted it now, in order to avoid further confusion. This is the current branch: gup_dma_core in that same git repo. It has the current set of call site conversions. Please note that there are a lot of conversions that are either incomplete or likely just plain wrong, at this point, but it is sufficient to at least boot up and run things such as fio(1). thanks,
From: John Hubbard <jhubbard@nvidia.com> Hi, It seems about time to post these initial patches: I think we have pretty good consensus on the concept and details of the put_user_pages() approach. Therefore, here are the first two patches, to get started on converting the get_user_pages() call sites to use put_user_page(), instead of put_page(). This is in order to implement tracking of get_user_page() pages. A discussion of the overall problem is below. As mentioned in patch 0001, the steps are to fix the problem are: 1) Provide put_user_page*() routines, intended to be used for releasing pages that were pinned via get_user_pages*(). 2) Convert all of the call sites for get_user_pages*(), to invoke put_user_page*(), instead of put_page(). This involves dozens of call sites, and will take some time. 3) After (2) is complete, use get_user_pages*() and put_user_page*() to implement tracking of these pages. This tracking will be separate from the existing struct page refcounting. 4) Use the tracking and identification of these pages, to implement special handling (especially in writeback paths) when the pages are backed by a filesystem. This write up is lifted from the RFC v2 patchset cover letter [1]: Overview ======== Some kernel components (file systems, device drivers) need to access memory that is specified via process virtual address. For a long time, the API to achieve that was get_user_pages ("GUP") and its variations. However, GUP has critical limitations that have been overlooked; in particular, GUP does not interact correctly with filesystems in all situations. That means that file-backed memory + GUP is a recipe for potential problems, some of which have already occurred in the field. GUP was first introduced for Direct IO (O_DIRECT), allowing filesystem code to get the struct page behind a virtual address and to let storage hardware perform a direct copy to or from that page. This is a short-lived access pattern, and as such, the window for a concurrent writeback of GUP'd page was small enough that there were not (we think) any reported problems. Also, userspace was expected to understand and accept that Direct IO was not synchronized with memory-mapped access to that data, nor with any process address space changes such as munmap(), mremap(), etc. Over the years, more GUP uses have appeared (virtualization, device drivers, RDMA) that can keep the pages they get via GUP for a long period of time (seconds, minutes, hours, days, ...). This long-term pinning makes an underlying design problem more obvious. In fact, there are a number of key problems inherent to GUP: Interactions with file systems ============================== File systems expect to be able to write back data, both to reclaim pages, and for data integrity. Allowing other hardware (NICs, GPUs, etc) to gain write access to the file memory pages means that such hardware can dirty the pages, without the filesystem being aware. This can, in some cases (depending on filesystem, filesystem options, block device, block device options, and other variables), lead to data corruption, and also to kernel bugs of the form: kernel BUG at /build/linux-fQ94TU/linux-4.4.0/fs/ext4/inode.c:1899! backtrace: ext4_writepage __writepage write_cache_pages ext4_writepages do_writepages __writeback_single_inode writeback_sb_inodes __writeback_inodes_wb wb_writeback wb_workfn process_one_work worker_thread kthread ret_from_fork ...which is due to the file system asserting that there are still buffer heads attached: ({ \ BUG_ON(!PagePrivate(page)); \ ((struct buffer_head *)page_private(page)); \ }) Dave Chinner's description of this is very clear: "The fundamental issue is that ->page_mkwrite must be called on every write access to a clean file backed page, not just the first one. How long the GUP reference lasts is irrelevant, if the page is clean and you need to dirty it, you must call ->page_mkwrite before it is marked writeable and dirtied. Every. Time." This is just one symptom of the larger design problem: filesystems do not actually support get_user_pages() being called on their pages, and letting hardware write directly to those pages--even though that pattern has been going on since about 2005 or so. Long term GUP ============= Long term GUP is an issue when FOLL_WRITE is specified to GUP (so, a writeable mapping is created), and the pages are file-backed. That can lead to filesystem corruption. What happens is that when a file-backed page is being written back, it is first mapped read-only in all of the CPU page tables; the file system then assumes that nobody can write to the page, and that the page content is therefore stable. Unfortunately, the GUP callers generally do not monitor changes to the CPU pages tables; they instead assume that the following pattern is safe (it's not): get_user_pages() Hardware can keep a reference to those pages for a very long time, and write to it at any time. Because "hardware" here means "devices that are not a CPU", this activity occurs without any interaction with the kernel's file system code. for each page set_page_dirty put_page() In fact, the GUP documentation even recommends that pattern. Anyway, the file system assumes that the page is stable (nothing is writing to the page), and that is a problem: stable page content is necessary for many filesystem actions during writeback, such as checksum, encryption, RAID striping, etc. Furthermore, filesystem features like COW (copy on write) or snapshot also rely on being able to use a new page for as memory for that memory range inside the file. Corruption during write back is clearly possible here. To solve that, one idea is to identify pages that have active GUP, so that we can use a bounce page to write stable data to the filesystem. The filesystem would work on the bounce page, while any of the active GUP might write to the original page. This would avoid the stable page violation problem, but note that it is only part of the overall solution, because other problems remain. Other filesystem features that need to replace the page with a new one can be inhibited for pages that are GUP-pinned. This will, however, alter and limit some of those filesystem features. The only fix for that would be to require GUP users to monitor and respond to CPU page table updates. Subsystems such as ODP and HMM do this, for example. This aspect of the problem is still under discussion. Direct IO ========= Direct IO can cause corruption, if userspace does Direct-IO that writes to a range of virtual addresses that are mmap'd to a file. The pages written to are file-backed pages that can be under write back, while the Direct IO is taking place. Here, Direct IO races with a write back: it calls GUP before page_mkclean() has replaced the CPU pte with a read-only entry. The race window is pretty small, which is probably why years have gone by before we noticed this problem: Direct IO is generally very quick, and tends to finish up before the filesystem gets around to do anything with the page contents. However, it's still a real problem. The solution is to never let GUP return pages that are under write back, but instead, force GUP to take a write fault on those pages. That way, GUP will properly synchronize with the active write back. This does not change the required GUP behavior, it just avoids that race. [1] https://lkml.kernel.org/r/20190204052135.25784-1-jhubbard@nvidia.com Cc: Christian Benvenuti <benve@cisco.com> Cc: Christoph Hellwig <hch@infradead.org> Cc: Christopher Lameter <cl@linux.com> Cc: Dan Williams <dan.j.williams@intel.com> Cc: Dave Chinner <david@fromorbit.com> Cc: Dennis Dalessandro <dennis.dalessandro@intel.com> Cc: Doug Ledford <dledford@redhat.com> Cc: Jan Kara <jack@suse.cz> Cc: Jason Gunthorpe <jgg@ziepe.ca> Cc: Jérôme Glisse <jglisse@redhat.com> Cc: Matthew Wilcox <willy@infradead.org> Cc: Michal Hocko <mhocko@kernel.org> Cc: Mike Rapoport <rppt@linux.ibm.com> Cc: Mike Marciniszyn <mike.marciniszyn@intel.com> Cc: Ralph Campbell <rcampbell@nvidia.com> Cc: Tom Talpey <tom@talpey.com> John Hubbard (2): mm: introduce put_user_page*(), placeholder versions infiniband/mm: convert put_page() to put_user_page*() drivers/infiniband/core/umem.c | 7 +- drivers/infiniband/core/umem_odp.c | 2 +- drivers/infiniband/hw/hfi1/user_pages.c | 11 +-- drivers/infiniband/hw/mthca/mthca_memfree.c | 6 +- drivers/infiniband/hw/qib/qib_user_pages.c | 11 +-- drivers/infiniband/hw/qib/qib_user_sdma.c | 6 +- drivers/infiniband/hw/usnic/usnic_uiom.c | 7 +- include/linux/mm.h | 24 ++++++ mm/swap.c | 82 +++++++++++++++++++++ 9 files changed, 129 insertions(+), 27 deletions(-)