Subject: Re: [PATCH v2] fs/dax: deposit pagetable even when installing zero page

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Wed, 13 Mar 2019 10:17:17 +0530

Hi Dan/Andrew/Jan,

"Aneesh Kumar K.V" <[email-masked]> writes:

> Architectures like ppc64 use the deposited page table to store hardware
> page table slot information. Make sure we deposit a page table when
> using zero page at the pmd level for hash.
>
> Without this we hit
>
> Unable to handle kernel paging request for data at address 0x00000000
> Faulting instruction address: 0xc000000000082a74
> Oops: Kernel access of bad area, sig: 11 [#1]
> ....
>
> NIP [c000000000082a74] __hash_page_thp+0x224/0x5b0
> LR [c0000000000829a4] __hash_page_thp+0x154/0x5b0
> Call Trace:
>  hash_page_mm+0x43c/0x740
>  do_hash_page+0x2c/0x3c
>  copy_from_iter_flushcache+0xa4/0x4a0
>  pmem_copy_from_iter+0x2c/0x50 [nd_pmem]
>  dax_copy_from_iter+0x40/0x70
>  dax_iomap_actor+0x134/0x360
>  iomap_apply+0xfc/0x1b0
>  dax_iomap_rw+0xac/0x130
>  ext4_file_write_iter+0x254/0x460 [ext4]
>  __vfs_write+0x120/0x1e0
>  vfs_write+0xd8/0x220
>  SyS_write+0x6c/0x110
>  system_call+0x3c/0x130
>
> Fixes: b5beae5e224f ("powerpc/pseries: Add driver for PAPR SCM regions")
> Reviewed-by: Jan Kara <[email-masked]>
> Signed-off-by: Aneesh Kumar K.V <[email-masked]>

Any suggestion on which tree this patch should got to? Also since this
fix a kernel crash, we may want to get this to 5.1?

> ---
> Changes from v1:
> * Add reviewed-by:
> * Add Fixes:
>
>  fs/dax.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 6959837cc465..01bfb2ac34f9 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -33,6 +33,7 @@
>  #include <linux/sizes.h>
>  #include <linux/mmu_notifier.h>
>  #include <linux/iomap.h>
> +#include <asm/pgalloc.h>
>  #include "internal.h"
>  
>  #define CREATE_TRACE_POINTS
> @@ -1410,7 +1411,9 @@ static vm_fault_t dax_pmd_load_hole(struct xa_state *xas, struct vm_fault *vmf,
>  {
>  	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
>  	unsigned long pmd_addr = vmf->address & PMD_MASK;
> +	struct vm_area_struct *vma = vmf->vma;
>  	struct inode *inode = mapping->host;
> +	pgtable_t pgtable = NULL;
>  	struct page *zero_page;
>  	spinlock_t *ptl;
>  	pmd_t pmd_entry;
> @@ -1425,12 +1428,22 @@ static vm_fault_t dax_pmd_load_hole(struct xa_state *xas, struct vm_fault *vmf,
>  	*entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn,
>  			DAX_PMD | DAX_ZERO_PAGE, false);
>  
> +	if (arch_needs_pgtable_deposit()) {
> +		pgtable = pte_alloc_one(vma->vm_mm);
> +		if (!pgtable)
> +			return VM_FAULT_OOM;
> +	}
> +
>  	ptl = pmd_lock(vmf->vma->vm_mm, vmf->pmd);
>  	if (!pmd_none(*(vmf->pmd))) {
>  		spin_unlock(ptl);
>  		goto fallback;
>  	}
>  
> +	if (pgtable) {
> +		pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
> +		mm_inc_nr_ptes(vma->vm_mm);
> +	}
>  	pmd_entry = mk_pmd(zero_page, vmf->vma->vm_page_prot);
>  	pmd_entry = pmd_mkhuge(pmd_entry);
>  	set_pmd_at(vmf->vma->vm_mm, pmd_addr, vmf->pmd, pmd_entry);
> @@ -1439,6 +1452,8 @@ static vm_fault_t dax_pmd_load_hole(struct xa_state *xas, struct vm_fault *vmf,
>  	return VM_FAULT_NOPAGE;
>  
>  fallback:
> +	if (pgtable)
> +		pte_free(vma->vm_mm, pgtable);
>  	trace_dax_pmd_load_hole_fallback(inode, vmf, zero_page, *entry);
>  	return VM_FAULT_FALLBACK;
>  }
> -- 
> 2.20.1

-aneesh