Re: [PATCH 2/2] mm/hugetlb: add wrapper functions for interactions with page cache

From: Sidhartha Kumar
Date: Fri Jun 16 2023 - 14:53:25 EST

On 6/15/23 4:36 PM, Mike Kravetz wrote:
On 06/09/23 12:52, Sidhartha Kumar wrote:
On 6/9/23 12:49 PM, Sidhartha Kumar wrote:
Signed-off-by: Sidhartha Kumar <sidhartha.kumar@xxxxxxxxxx>

Sorry, I missed adding the commit message to this. It should be:

Add filemap_lock_hugetlb_folio() which is wraps __filemap_get_folio()
and passes in a linear page index. hugetlb_add_to_page_cache() is modified
to also compute a linear page index before calling into page cache code.

linear_page_index() is modified to perform the computation on hugetlb
so we can use it in the page cache wrappers.

---
fs/hugetlbfs/inode.c | 14 +++++++-------
include/linux/hugetlb.h | 21 +++++++++++++++++++--
include/linux/pagemap.h | 2 --
mm/hugetlb.c | 22 +++++++++++++---------
4 files changed, 39 insertions(+), 20 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 90361a922cec7..90d27a8af4b6a 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -617,20 +617,19 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart,
struct hstate *h = hstate_inode(inode);
struct address_space *mapping = &inode->i_data;
const pgoff_t start = lstart >> huge_page_shift(h);
- const pgoff_t end = lend >> huge_page_shift(h);
struct folio_batch fbatch;
pgoff_t next, index;
int i, freed = 0;
bool truncate_op = (lend == LLONG_MAX);
folio_batch_init(&fbatch);
- next = start;
- while (filemap_get_folios(mapping, &next, end - 1, &fbatch)) {
+ next = lstart;
+ while (filemap_get_folios(mapping, &next, lend - 1, &fbatch)) {

This does not seem correct. At this point next == lstart which is a file
offset passed to the routine as opposed to an index.

I would think next needs to be set to 'lstart >> PAGE_SHIFT' here.

for (i = 0; i < folio_batch_count(&fbatch); ++i) {
struct folio *folio = fbatch.folios[i];
u32 hash = 0;
- index = folio->index;
+ index = (folio->index) >> huge_page_shift(h);

Here you want to convert index from the PAGE_SIZE index to a hugetlb
page size index. Correct?
I am terrible at arithmetic, but huge_page_shift already includes
PAGE_SHIFT, so it seems like you want this to me.

index = (folio->index) >> huge_page_order(h);

hash = hugetlb_fault_mutex_hash(mapping, index);
mutex_lock(&hugetlb_fault_mutex_table[hash]);
@@ -693,10 +692,11 @@ static void hugetlbfs_zero_partial_page(struct hstate *h,
loff_t start,
loff_t end)
{
- pgoff_t idx = start >> huge_page_shift(h);
+ struct mm_struct *mm = current->mm;
+ struct vm_area_struct *vma = find_vma(mm, start);
struct folio *folio;
- folio = filemap_lock_folio(mapping, idx);
+ folio = filemap_lock_hugetlb_folio(vma, start);

Here you are passing an address/index that may be associated with a
tail page. I assume since the hugetlb folio is multi-order, the
returned folio will be for the 'head page'. Correct?

if (IS_ERR(folio))
return;
@@ -868,7 +868,7 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
}
clear_huge_page(&folio->page, addr, pages_per_huge_page(h));
__folio_mark_uptodate(folio);
- error = hugetlb_add_to_page_cache(folio, mapping, index);
+ error = hugetlb_add_to_page_cache(folio, &pseudo_vma, mapping, addr);
if (unlikely(error)) {
restore_reserve_on_error(h, &pseudo_vma, addr, folio);
folio_put(folio);
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 21f942025fecd..55f90e051b7a2 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -727,8 +727,8 @@ struct folio *alloc_hugetlb_folio_nodemask(struct hstate *h, int preferred_nid,
nodemask_t *nmask, gfp_t gfp_mask);
struct folio *alloc_hugetlb_folio_vma(struct hstate *h, struct vm_area_struct *vma,
unsigned long address);
-int hugetlb_add_to_page_cache(struct folio *folio, struct address_space *mapping,
- pgoff_t idx);
+int hugetlb_add_to_page_cache(struct folio *folio, struct vm_area_struct *vma,
+ struct address_space *mapping, unsigned long address);
void restore_reserve_on_error(struct hstate *h, struct vm_area_struct *vma,
unsigned long address, struct folio *folio);
@@ -755,6 +755,16 @@ static inline struct hugepage_subpool *hugetlb_folio_subpool(struct folio *folio
return folio->_hugetlb_subpool;
}
+/* Wrapper function for __filemap_get_folio*/
+static inline struct folio *filemap_lock_hugetlb_folio(struct vm_area_struct *vma,
+ unsigned long address)
+{
+ struct address_space *mapping = vma->vm_file->f_mapping;
+
+ pgoff_t idx = linear_page_index(vma, address);
+ return __filemap_get_folio(mapping, idx, FGP_LOCK, 0);
+}

I like the wrapper idea. This is going to replace existing calls to
filemap_lock_folio. What about something like this for the routine:

static inline struct folio *filemap_lock_hugetlb_folio(struct hstate *h,
struct address_space *mapping,
pgoff_t index);
{
/* please verify my arithmetic */
return filemap_lock_folio(mapping, index << huge_page_order(h));
}

In this way, existing callers would only need to be changed to pass in
hstate. Perhaps, there was a reason for your wrapper not obvious to me?

+
static inline void hugetlb_set_folio_subpool(struct folio *folio,
struct hugepage_subpool *subpool)
{
@@ -1021,6 +1031,13 @@ static inline struct hugepage_subpool *hugetlb_folio_subpool(struct folio *folio
return NULL;
}
+/* Wrapper function for __filemap_get_folio*/
+static inline struct folio *filemap_lock_hugetlb_folio(struct vm_area_struct *vma,
+ unsigned long address)
+{
+ return NULL;
+}
+
static inline int isolate_or_dissolve_huge_page(struct page *page,
struct list_head *list)
{
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 17c414fc2136e..ae8f36966d7b3 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -860,8 +860,6 @@ static inline pgoff_t linear_page_index(struct vm_area_struct *vma,
unsigned long address)
{
pgoff_t pgoff;
- if (unlikely(is_vm_hugetlb_page(vma)))
- return linear_hugepage_index(vma, address);
pgoff = (address - vma->vm_start) >> PAGE_SHIFT;
pgoff += vma->vm_pgoff;
return pgoff;
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index dfa412d8cb300..824d6d215a161 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -951,7 +951,7 @@ static long region_count(struct resv_map *resv, long f, long t)
/*
* Convert the address within this vma to the page offset within
- * the mapping, in pagecache page units; huge pages here.
+ * the mapping, in huge page units here.
*/
static pgoff_t vma_hugecache_offset(struct hstate *h,
struct vm_area_struct *vma, unsigned long address)
@@ -5730,7 +5730,7 @@ static bool hugetlbfs_pagecache_present(struct hstate *h,
struct vm_area_struct *vma, unsigned long address)
{
struct address_space *mapping = vma->vm_file->f_mapping;
- pgoff_t idx = vma_hugecache_offset(h, vma, address);
+ pgoff_t idx = linear_page_index(vma, address);
bool present;
rcu_read_lock();
@@ -5740,13 +5740,16 @@ static bool hugetlbfs_pagecache_present(struct hstate *h,
return present;
}
-int hugetlb_add_to_page_cache(struct folio *folio, struct address_space *mapping,
- pgoff_t idx)
+int hugetlb_add_to_page_cache(struct folio *folio,
+ struct vm_area_struct *vma,
+ struct address_space *mapping,
+ unsigned long address)
{

Like filemap_lock_hugetlb_folio(), can you just add hstate to existing
hugetlb_add_to_page_cache() arguments and do arithmetic to convert index
to PAGE_SIZE based index?


I wanted to avoid using "magic numbers" through doing arithmetic shifts to convert the index and instead explicitly use the linear_page_index() helper function. However this does add more complexity as the function parameters are changed. I can change it to doing the arithmetic shift to minimize code changes for v2.

Thanks,
Sidhartha Kumar

Again, I could be missing something. But, IMO such a conversion would
minimize changes to the current code.