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mm: hold the RCU read lock over calls to ->map_pages 

Prevent filesystems from doing things which sleep in their map_pages
method.  This is in preparation for a pagefault path protected only by
RCU.

Link: https://lkml.kernel.org/r/20230327174515.1811532-4-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Matthew Wilcox (Oracle) 2023-03-27 18:45:15 +01:00 committed by Andrew Morton
parent 0050d7f5ee
commit 58ef47ef7d
2 changed files with 10 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
Documentation/filesystems/locking.rst
View File

@ -645,7 +645,7 @@ ops mmap_lock PageLocked(page)
open: yes
close: yes
fault: yes can return with page locked
map_pages: yes
map_pages: read
page_mkwrite: yes can return with page locked
pfn_mkwrite: yes
access: yes
@ -661,7 +661,7 @@ locked. The VM will unlock the page.
->map_pages() is called when VM asks to map easy accessible pages.
Filesystem should find and map pages associated with offsets from "start_pgoff"
till "end_pgoff". ->map_pages() is called with page table locked and must
till "end_pgoff". ->map_pages() is called with the RCU lock held and must
not block. If it's not possible to reach a page without blocking,
filesystem should skip it. Filesystem should use do_set_pte() to setup
page table entry. Pointer to entry associated with the page is passed in

11
mm/memory.c
View File

@ -4450,6 +4450,7 @@ static vm_fault_t do_fault_around(struct vm_fault *vmf)
/* The page offset of vmf->address within the VMA. */
pgoff_t vma_off = vmf->pgoff - vmf->vma->vm_pgoff;
pgoff_t from_pte, to_pte;
vm_fault_t ret;
/* The PTE offset of the start address, clamped to the VMA. */
from_pte = max(ALIGN_DOWN(pte_off, nr_pages),
@ -4465,9 +4466,13 @@ static vm_fault_t do_fault_around(struct vm_fault *vmf)
return VM_FAULT_OOM;
}
return vmf->vma->vm_ops->map_pages(vmf,
vmf->pgoff + from_pte - pte_off,
vmf->pgoff + to_pte - pte_off);
rcu_read_lock();
ret = vmf->vma->vm_ops->map_pages(vmf,
vmf->pgoff + from_pte - pte_off,
vmf->pgoff + to_pte - pte_off);
rcu_read_unlock();
return ret;
}
/* Return true if we should do read fault-around, false otherwise */