Commit bc2466e4 authored by Jan Kara's avatar Jan Kara Committed by Ross Zwisler
Browse files

dax: Use radix tree entry lock to protect cow faults 



When doing cow faults, we cannot directly fill in PTE as we do for other
faults as we rely on generic code to do proper accounting of the cowed page.
We also have no page to lock to protect against races with truncate as
other faults have and we need the protection to extend until the moment
generic code inserts cowed page into PTE thus at that point we have no
protection of fs-specific i_mmap_sem. So far we relied on using
i_mmap_lock for the protection however that is completely special to cow
faults. To make fault locking more uniform use DAX entry lock instead.

Reviewed-by: default avatarRoss Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: default avatarJan Kara <jack@suse.cz>
Signed-off-by: default avatarRoss Zwisler <ross.zwisler@linux.intel.com>
parent ac401cc7

fs/dax.c

+5 −7
Original line number Diff line number Diff line
@@ -478,7 +478,7 @@ void dax_wake_mapping_entry_waiter(struct address_space *mapping, 
	}

}



static void unlock_mapping_entry(struct address_space *mapping, pgoff_t index)

void dax_unlock_mapping_entry(struct address_space *mapping, pgoff_t index)

{

	void *ret, **slot;
@@ -501,7 +501,7 @@ static void put_locked_mapping_entry(struct address_space *mapping, 
		unlock_page(entry);

		put_page(entry);

	} else {

		unlock_mapping_entry(mapping, index);

		dax_unlock_mapping_entry(mapping, index);

	}

}
@@ -884,13 +884,11 @@ int __dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf, 
			goto unlock_entry;

		if (!radix_tree_exceptional_entry(entry)) {

			vmf->page = entry;

		} else {

			unlock_mapping_entry(mapping, vmf->pgoff);

			i_mmap_lock_read(mapping);

			vmf->page = NULL;

		}

			return VM_FAULT_LOCKED;

		}

		vmf->entry = entry;

		return VM_FAULT_DAX_LOCKED;

	}



	if (!buffer_mapped(&bh)) {

		if (vmf->flags & FAULT_FLAG_WRITE) {

include/linux/dax.h

+7 −0
Original line number Diff line number Diff line
@@ -21,6 +21,7 @@ void dax_wake_mapping_entry_waiter(struct address_space *mapping, 


#ifdef CONFIG_FS_DAX

struct page *read_dax_sector(struct block_device *bdev, sector_t n);

void dax_unlock_mapping_entry(struct address_space *mapping, pgoff_t index);

int __dax_zero_page_range(struct block_device *bdev, sector_t sector,

		unsigned int offset, unsigned int length);

#else
@@ -29,6 +30,12 @@ static inline struct page *read_dax_sector(struct block_device *bdev, 
{

	return ERR_PTR(-ENXIO);

}

/* Shouldn't ever be called when dax is disabled. */

static inline void dax_unlock_mapping_entry(struct address_space *mapping,

					    pgoff_t index)

{

	BUG();

}

static inline int __dax_zero_page_range(struct block_device *bdev,

		sector_t sector, unsigned int offset, unsigned int length)

{

include/linux/mm.h

+7 −0
Original line number Diff line number Diff line
@@ -299,6 +299,12 @@ struct vm_fault { 
					 * is set (which is also implied by

					 * VM_FAULT_ERROR).

					 */

	void *entry;			/* ->fault handler can alternatively

					 * return locked DAX entry. In that

					 * case handler should return

					 * VM_FAULT_DAX_LOCKED and fill in

					 * entry here.

					 */

	/* for ->map_pages() only */

	pgoff_t max_pgoff;		/* map pages for offset from pgoff till

					 * max_pgoff inclusive */
@@ -1084,6 +1090,7 @@ static inline void clear_page_pfmemalloc(struct page *page) 
#define VM_FAULT_LOCKED	0x0200	/* ->fault locked the returned page */

#define VM_FAULT_RETRY	0x0400	/* ->fault blocked, must retry */

#define VM_FAULT_FALLBACK 0x0800	/* huge page fault failed, fall back to small */

#define VM_FAULT_DAX_LOCKED 0x1000	/* ->fault has locked DAX entry */



#define VM_FAULT_HWPOISON_LARGE_MASK 0xf000 /* encodes hpage index for large hwpoison */

mm/memory.c

+18 −20
Original line number Diff line number Diff line
@@ -63,6 +63,7 @@ 
#include <linux/dma-debug.h>

#include <linux/debugfs.h>

#include <linux/userfaultfd_k.h>

#include <linux/dax.h>



#include <asm/io.h>

#include <asm/mmu_context.h>
@@ -2785,7 +2786,8 @@ oom: 
 */

static int __do_fault(struct vm_area_struct *vma, unsigned long address,

			pgoff_t pgoff, unsigned int flags,

			struct page *cow_page, struct page **page)

			struct page *cow_page, struct page **page,

			void **entry)

{

	struct vm_fault vmf;

	int ret;
@@ -2800,8 +2802,10 @@ static int __do_fault(struct vm_area_struct *vma, unsigned long address, 
	ret = vma->vm_ops->fault(vma, &vmf);

	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))

		return ret;

	if (!vmf.page)

		goto out;

	if (ret & VM_FAULT_DAX_LOCKED) {

		*entry = vmf.entry;

		return ret;

	}



	if (unlikely(PageHWPoison(vmf.page))) {

		if (ret & VM_FAULT_LOCKED)
@@ -2815,7 +2819,6 @@ static int __do_fault(struct vm_area_struct *vma, unsigned long address, 
	else

		VM_BUG_ON_PAGE(!PageLocked(vmf.page), vmf.page);



 out:

	*page = vmf.page;

	return ret;

}
@@ -2987,7 +2990,7 @@ static int do_read_fault(struct mm_struct *mm, struct vm_area_struct *vma, 
		pte_unmap_unlock(pte, ptl);

	}



	ret = __do_fault(vma, address, pgoff, flags, NULL, &fault_page);

	ret = __do_fault(vma, address, pgoff, flags, NULL, &fault_page, NULL);

	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))

		return ret;
@@ -3010,6 +3013,7 @@ static int do_cow_fault(struct mm_struct *mm, struct vm_area_struct *vma, 
		pgoff_t pgoff, unsigned int flags, pte_t orig_pte)

{

	struct page *fault_page, *new_page;

	void *fault_entry;

	struct mem_cgroup *memcg;

	spinlock_t *ptl;

	pte_t *pte;
@@ -3027,26 +3031,24 @@ static int do_cow_fault(struct mm_struct *mm, struct vm_area_struct *vma, 
		return VM_FAULT_OOM;

	}



	ret = __do_fault(vma, address, pgoff, flags, new_page, &fault_page);

	ret = __do_fault(vma, address, pgoff, flags, new_page, &fault_page,

			 &fault_entry);

	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))

		goto uncharge_out;



	if (fault_page)

	if (!(ret & VM_FAULT_DAX_LOCKED))

		copy_user_highpage(new_page, fault_page, address, vma);

	__SetPageUptodate(new_page);



	pte = pte_offset_map_lock(mm, pmd, address, &ptl);

	if (unlikely(!pte_same(*pte, orig_pte))) {

		pte_unmap_unlock(pte, ptl);

		if (fault_page) {

		if (!(ret & VM_FAULT_DAX_LOCKED)) {

			unlock_page(fault_page);

			put_page(fault_page);

		} else {

			/*

			 * The fault handler has no page to lock, so it holds

			 * i_mmap_lock for read to protect against truncate.

			 */

			i_mmap_unlock_read(vma->vm_file->f_mapping);

			dax_unlock_mapping_entry(vma->vm_file->f_mapping,

						 pgoff);

		}

		goto uncharge_out;

	}
@@ -3054,15 +3056,11 @@ static int do_cow_fault(struct mm_struct *mm, struct vm_area_struct *vma, 
	mem_cgroup_commit_charge(new_page, memcg, false, false);

	lru_cache_add_active_or_unevictable(new_page, vma);

	pte_unmap_unlock(pte, ptl);

	if (fault_page) {

	if (!(ret & VM_FAULT_DAX_LOCKED)) {

		unlock_page(fault_page);

		put_page(fault_page);

	} else {

		/*

		 * The fault handler has no page to lock, so it holds

		 * i_mmap_lock for read to protect against truncate.

		 */

		i_mmap_unlock_read(vma->vm_file->f_mapping);

		dax_unlock_mapping_entry(vma->vm_file->f_mapping, pgoff);

	}

	return ret;

uncharge_out:
@@ -3082,7 +3080,7 @@ static int do_shared_fault(struct mm_struct *mm, struct vm_area_struct *vma, 
	int dirtied = 0;

	int ret, tmp;



	ret = __do_fault(vma, address, pgoff, flags, NULL, &fault_page);

	ret = __do_fault(vma, address, pgoff, flags, NULL, &fault_page, NULL);

	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))

		return ret;

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