Re: [PATCH v3 02/10] fsdax: Factor helper: dax_fault_actor()
From: Ritesh Harjani
Date: Tue Mar 23 2021 - 11:59:31 EST
On 3/19/21 7:22 AM, Shiyang Ruan wrote:
The core logic in the two dax page fault functions is similar. So, move
the logic into a common helper function. Also, to facilitate the
addition of new features, such as CoW, switch-case is no longer used to
handle different iomap types.
Signed-off-by: Shiyang Ruan <ruansy.fnst@xxxxxxxxxxx>
---
fs/dax.c | 291 +++++++++++++++++++++++++++----------------------------
1 file changed, 145 insertions(+), 146 deletions(-)
diff --git a/fs/dax.c b/fs/dax.c
index 7031e4302b13..33ddad0f3091 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1053,6 +1053,66 @@ static vm_fault_t dax_load_hole(struct xa_state *xas,
return ret;
}
+#ifdef CONFIG_FS_DAX_PMD
+static vm_fault_t dax_pmd_load_hole(struct xa_state *xas, struct vm_fault *vmf,
+ struct iomap *iomap, void **entry)
+{
+ 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;
+ pfn_t pfn;
+
+ zero_page = mm_get_huge_zero_page(vmf->vma->vm_mm);
+
+ if (unlikely(!zero_page))
+ goto fallback;
+
+ pfn = page_to_pfn_t(zero_page);
+ *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);
+ spin_unlock(ptl);
+ trace_dax_pmd_load_hole(inode, vmf, zero_page, *entry);
+ 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;
+}
+#else
+static vm_fault_t dax_pmd_load_hole(struct xa_state *xas, struct vm_fault *vmf,
+ struct iomap *iomap, void **entry)
+{
+ return VM_FAULT_FALLBACK;
+}
+#endif /* CONFIG_FS_DAX_PMD */
+
s64 dax_iomap_zero(loff_t pos, u64 length, struct iomap *iomap)
{
sector_t sector = iomap_sector(iomap, pos & PAGE_MASK);
@@ -1289,6 +1349,61 @@ static int dax_fault_cow_page(struct vm_fault *vmf, struct iomap *iomap,
return 0;
}
+/**
+ * dax_fault_actor - Common actor to handle pfn insertion in PTE/PMD fault.
+ * @vmf: vm fault instance
+ * @pfnp: pfn to be returned
+ * @xas: the dax mapping tree of a file
+ * @entry: an unlocked dax entry to be inserted
+ * @pmd: distinguish whether it is a pmd fault
+ * @flags: iomap flags
+ * @iomap: from iomap_begin()
+ * @srcmap: from iomap_begin(), not equal to iomap if it is a CoW
+ */
+static vm_fault_t dax_fault_actor(struct vm_fault *vmf, pfn_t *pfnp,
+ struct xa_state *xas, void *entry, bool pmd, unsigned int flags,
+ struct iomap *iomap, struct iomap *srcmap)
+{
+ struct address_space *mapping = vmf->vma->vm_file->f_mapping;
+ size_t size = pmd ? PMD_SIZE : PAGE_SIZE;
+ loff_t pos = (loff_t)xas->xa_offset << PAGE_SHIFT;
shouldn't we use xa_index here for pos ?
(loff_t)xas->xa_index << PAGE_SHIFT;
+ bool write = vmf->flags & FAULT_FLAG_WRITE;
+ bool sync = dax_fault_is_synchronous(flags, vmf->vma, iomap);
+ int err = 0;
+ pfn_t pfn;
+
+ /* if we are reading UNWRITTEN and HOLE, return a hole. */
+ if (!write &&
+ (iomap->type == IOMAP_UNWRITTEN || iomap->type == IOMAP_HOLE)) {
+ if (!pmd)
+ return dax_load_hole(xas, mapping, &entry, vmf);
+ else
+ return dax_pmd_load_hole(xas, vmf, iomap, &entry);
+ }
+
+ if (iomap->type != IOMAP_MAPPED) {
+ WARN_ON_ONCE(1);
+ return VM_FAULT_SIGBUS;
+ }
So now in case if mapping is not mapped, we always cause
VM_FAULT_SIGBUG. But earlier we were only doing WARN_ON_ONCE(1).
Can you pls help answer why the change in behavior?
+
+ err = dax_iomap_pfn(iomap, pos, size, &pfn);
+ if (err)
+ return dax_fault_return(err);
Same case here as well. This could return SIGBUS while earlier I am not
sure why were we only returning FALLBACK?
+
+ entry = dax_insert_entry(xas, mapping, vmf, entry, pfn, 0,
+ write && !sync);
In dax_insert_entry() we are passing 0 as flags.
We should be passing DAX_PMD/DAX_PTE no?
+
+ if (sync)
+ return dax_fault_synchronous_pfnp(pfnp, pfn);
+
/* handle PMD case here */
+ if (pmd)
+ return vmf_insert_pfn_pmd(vmf, pfn, write);
/* handle PTE case here */
+ if (write)
+ return vmf_insert_mixed_mkwrite(vmf->vma, vmf->address, pfn);
+ else
+ return vmf_insert_mixed(vmf->vma, vmf->address, pfn);
+}
It is easy to miss the return from if(pmd) case while reading.
A comment like above could be helpful for code review.
+
static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
int *iomap_errp, const struct iomap_ops *ops)
{
@@ -1296,17 +1411,14 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
struct address_space *mapping = vma->vm_file->f_mapping;
XA_STATE(xas, &mapping->i_pages, vmf->pgoff);
struct inode *inode = mapping->host;
- unsigned long vaddr = vmf->address;
loff_t pos = (loff_t)vmf->pgoff << PAGE_SHIFT;
struct iomap iomap = { .type = IOMAP_HOLE };
struct iomap srcmap = { .type = IOMAP_HOLE };
unsigned flags = IOMAP_FAULT;
int error, major = 0;
bool write = vmf->flags & FAULT_FLAG_WRITE;
- bool sync;
vm_fault_t ret = 0;
void *entry;
- pfn_t pfn;
trace_dax_pte_fault(inode, vmf, ret);
/*
@@ -1352,8 +1464,8 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
goto unlock_entry;
}
if (WARN_ON_ONCE(iomap.offset + iomap.length < pos + PAGE_SIZE)) {
- error = -EIO; /* fs corruption? */
- goto error_finish_iomap;
+ ret = VM_FAULT_SIGBUS; /* fs corruption? */
+ goto finish_iomap;
}
if (vmf->cow_page) {
@@ -1363,49 +1475,19 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
goto finish_iomap;
}
- sync = dax_fault_is_synchronous(flags, vma, &iomap);
-
- switch (iomap.type) {
- case IOMAP_MAPPED:
- if (iomap.flags & IOMAP_F_NEW) {
- count_vm_event(PGMAJFAULT);
- count_memcg_event_mm(vma->vm_mm, PGMAJFAULT);
- major = VM_FAULT_MAJOR;
- }
- error = dax_iomap_pfn(&iomap, pos, PAGE_SIZE, &pfn);
- if (error < 0)
- goto error_finish_iomap;
-
- entry = dax_insert_entry(&xas, mapping, vmf, entry, pfn,
- 0, write && !sync);
-
- if (sync) {
- ret = dax_fault_synchronous_pfnp(pfnp, pfn);
- goto finish_iomap;
- }
- trace_dax_insert_mapping(inode, vmf, entry);
- if (write)
- ret = vmf_insert_mixed_mkwrite(vma, vaddr, pfn);
- else
- ret = vmf_insert_mixed(vma, vaddr, pfn);
-
+ ret = dax_fault_actor(vmf, pfnp, &xas, entry, false, flags,
+ &iomap, &srcmap);
+ if (ret == VM_FAULT_SIGBUS)
goto finish_iomap;
- case IOMAP_UNWRITTEN:
- case IOMAP_HOLE:
- if (!write) {
- ret = dax_load_hole(&xas, mapping, &entry, vmf);
- goto finish_iomap;
- }
- fallthrough;
- default:
- WARN_ON_ONCE(1);
- error = -EIO;
- break;
+
+ /* read/write MAPPED, CoW UNWRITTEN */
+ if (iomap.flags & IOMAP_F_NEW) {
+ count_vm_event(PGMAJFAULT);
+ count_memcg_event_mm(vma->vm_mm, PGMAJFAULT);
+ major = VM_FAULT_MAJOR;
}
It is much better if above accounting is also done in dax_fault_actor()
function itself. Then at the end of this function we need to just do
"return ret" instead of "return ret | major"
- error_finish_iomap:
- ret = dax_fault_return(error);
- finish_iomap:
+finish_iomap:
if (ops->iomap_end) {
int copied = PAGE_SIZE;
@@ -1419,66 +1501,14 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
*/
ops->iomap_end(inode, pos, PAGE_SIZE, copied, flags, &iomap);
}
- unlock_entry:
+unlock_entry:
dax_unlock_entry(&xas, entry);
- out:
+out:
trace_dax_pte_fault_done(inode, vmf, ret);
return ret | major;
}
-ritesh