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Date: Mon, 13 Jul 2015 17:05:00 +0200
From: Jan Kara <jack@...e.cz>
To: Matthew Wilcox <matthew.r.wilcox@...el.com>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, Matthew Wilcox <willy@...ux.intel.com>
Subject: Re: [PATCH 07/10] dax: Add huge page fault support
On Fri 10-07-15 16:29:22, Matthew Wilcox wrote:
> From: Matthew Wilcox <willy@...ux.intel.com>
>
> This is the support code for DAX-enabled filesystems to allow them to
> provide huge pages in response to faults.
>
> Signed-off-by: Matthew Wilcox <willy@...ux.intel.com>
...
> +int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
> + pmd_t *pmd, unsigned int flags, get_block_t get_block,
> + dax_iodone_t complete_unwritten)
> +{
> + struct file *file = vma->vm_file;
> + struct address_space *mapping = file->f_mapping;
> + struct inode *inode = mapping->host;
> + struct buffer_head bh;
> + unsigned blkbits = inode->i_blkbits;
> + unsigned long pmd_addr = address & PMD_MASK;
> + bool write = flags & FAULT_FLAG_WRITE;
> + long length;
> + void *kaddr;
> + pgoff_t size, pgoff;
> + sector_t block, sector;
> + unsigned long pfn;
> + int result = 0;
> +
> + /* Fall back to PTEs if we're going to COW */
> + if (write && !(vma->vm_flags & VM_SHARED))
> + return VM_FAULT_FALLBACK;
> + /* If the PMD would extend outside the VMA */
> + if (pmd_addr < vma->vm_start)
> + return VM_FAULT_FALLBACK;
> + if ((pmd_addr + PMD_SIZE) > vma->vm_end)
> + return VM_FAULT_FALLBACK;
> +
> + pgoff = ((pmd_addr - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
> + size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
> + if (pgoff >= size)
> + return VM_FAULT_SIGBUS;
> + /* If the PMD would cover blocks out of the file */
> + if ((pgoff | PG_PMD_COLOUR) >= size)
> + return VM_FAULT_FALLBACK;
> +
> + memset(&bh, 0, sizeof(bh));
> + block = (sector_t)pgoff << (PAGE_SHIFT - blkbits);
> +
> + bh.b_size = PMD_SIZE;
> + length = get_block(inode, block, &bh, write);
> + if (length)
> + return VM_FAULT_SIGBUS;
> + i_mmap_lock_read(mapping);
> +
> + /*
> + * If the filesystem isn't willing to tell us the length of a hole,
> + * just fall back to PTEs. Calling get_block 512 times in a loop
> + * would be silly.
> + */
> + if (!buffer_size_valid(&bh) || bh.b_size < PMD_SIZE)
> + goto fallback;
> +
> + /* Guard against a race with truncate */
> + size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
> + if (pgoff >= size) {
> + result = VM_FAULT_SIGBUS;
> + goto out;
> + }
So if this is a writeable fault and we race with truncate, we can leave
stale blocks beyond i_size, can't we? Ah, looking at dax_insert_mapping()
this seems to be a documented quirk of DAX mmap code. Would be worth
mentioning here as well so that people don't wonder...
Otherwise the patch looks good to me.
Honza
> + if ((pgoff | PG_PMD_COLOUR) >= size)
> + goto fallback;
> +
> + if (is_huge_zero_pmd(*pmd))
> + unmap_mapping_range(mapping, pgoff << PAGE_SHIFT, PMD_SIZE, 0);
> +
> + if (!write && !buffer_mapped(&bh) && buffer_uptodate(&bh)) {
> + bool set;
> + spinlock_t *ptl;
> + struct mm_struct *mm = vma->vm_mm;
> + struct page *zero_page = get_huge_zero_page();
> + if (unlikely(!zero_page))
> + goto fallback;
> +
> + ptl = pmd_lock(mm, pmd);
> + set = set_huge_zero_page(NULL, mm, vma, pmd_addr, pmd,
> + zero_page);
> + spin_unlock(ptl);
> + result = VM_FAULT_NOPAGE;
> + } else {
> + sector = bh.b_blocknr << (blkbits - 9);
> + length = bdev_direct_access(bh.b_bdev, sector, &kaddr, &pfn,
> + bh.b_size);
> + if (length < 0) {
> + result = VM_FAULT_SIGBUS;
> + goto out;
> + }
> + if ((length < PMD_SIZE) || (pfn & PG_PMD_COLOUR))
> + goto fallback;
> +
> + if (buffer_unwritten(&bh) || buffer_new(&bh)) {
> + int i;
> + for (i = 0; i < PTRS_PER_PMD; i++)
> + clear_page(kaddr + i * PAGE_SIZE);
> + count_vm_event(PGMAJFAULT);
> + mem_cgroup_count_vm_event(vma->vm_mm, PGMAJFAULT);
> + result |= VM_FAULT_MAJOR;
> + }
> +
> + result |= vmf_insert_pfn_pmd(vma, address, pmd, pfn, write);
> + }
> +
> + out:
> + i_mmap_unlock_read(mapping);
> +
> + if (buffer_unwritten(&bh))
> + complete_unwritten(&bh, !(result & VM_FAULT_ERROR));
> +
> + return result;
> +
> + fallback:
> + count_vm_event(THP_FAULT_FALLBACK);
> + result = VM_FAULT_FALLBACK;
> + goto out;
> +}
> +EXPORT_SYMBOL_GPL(__dax_pmd_fault);
> +
> +/**
> + * dax_pmd_fault - handle a PMD fault on a DAX file
> + * @vma: The virtual memory area where the fault occurred
> + * @vmf: The description of the fault
> + * @get_block: The filesystem method used to translate file offsets to blocks
> + *
> + * When a page fault occurs, filesystems may call this helper in their
> + * pmd_fault handler for DAX files.
> + */
> +int dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
> + pmd_t *pmd, unsigned int flags, get_block_t get_block,
> + dax_iodone_t complete_unwritten)
> +{
> + int result;
> + struct super_block *sb = file_inode(vma->vm_file)->i_sb;
> +
> + if (flags & FAULT_FLAG_WRITE) {
> + sb_start_pagefault(sb);
> + file_update_time(vma->vm_file);
> + }
> + result = __dax_pmd_fault(vma, address, pmd, flags, get_block,
> + complete_unwritten);
> + if (flags & FAULT_FLAG_WRITE)
> + sb_end_pagefault(sb);
> +
> + return result;
> +}
> +EXPORT_SYMBOL_GPL(dax_pmd_fault);
> +#endif /* CONFIG_TRANSPARENT_HUGEPAGES */
> +
> /**
> * dax_pfn_mkwrite - handle first write to DAX page
> * @vma: The virtual memory area where the fault occurred
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index 9b51f9d..b415e52 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -14,6 +14,20 @@ int dax_fault(struct vm_area_struct *, struct vm_fault *, get_block_t,
> dax_iodone_t);
> int __dax_fault(struct vm_area_struct *, struct vm_fault *, get_block_t,
> dax_iodone_t);
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +int dax_pmd_fault(struct vm_area_struct *, unsigned long addr, pmd_t *,
> + unsigned int flags, get_block_t, dax_iodone_t);
> +int __dax_pmd_fault(struct vm_area_struct *, unsigned long addr, pmd_t *,
> + unsigned int flags, get_block_t, dax_iodone_t);
> +#else
> +static inline int dax_pmd_fault(struct vm_area_struct *vma, unsigned long addr,
> + pmd_t *pmd, unsigned int flags, get_block_t gb,
> + dax_iodone_t di)
> +{
> + return VM_FAULT_FALLBACK;
> +}
> +#define __dax_pmd_fault dax_pmd_fault
> +#endif
> int dax_pfn_mkwrite(struct vm_area_struct *, struct vm_fault *);
> #define dax_mkwrite(vma, vmf, gb, iod) dax_fault(vma, vmf, gb, iod)
> #define __dax_mkwrite(vma, vmf, gb, iod) __dax_fault(vma, vmf, gb, iod)
> --
> 2.1.4
>
> --
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--
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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