Re: [PATCH 00/21] Folio patches for 5.19

[Nathan Chancellor <nathan@kernel.org>]

On Fri, Apr 29, 2022 at 08:23:08PM +0100, Matthew Wilcox (Oracle) wrote:
> Andrew, do you want to include these patches in -mm?
> 
>  - Finish the conversion from alloc_pages_vma() to vma_alloc_folio()
>  - Finish converting shrink_page_list() to folios
>  - Start converting shmem from pages to folios (alas, not finished,
>    I have simply run out of time with all the debugging/fixing needed
>    for 5.18)
> 
> Matthew Wilcox (Oracle) (21):
>   shmem: Convert shmem_alloc_hugepage() to use vma_alloc_folio()
>   mm/huge_memory: Convert do_huge_pmd_anonymous_page() to use 
>     vma_alloc_folio()
>   mm: Remove alloc_pages_vma()
>   vmscan: Use folio_mapped() in shrink_page_list()
>   vmscan: Convert the writeback handling in shrink_page_list() to folios
>   swap: Turn get_swap_page() into folio_alloc_swap()
>   swap: Convert add_to_swap() to take a folio
>   vmscan: Convert dirty page handling to folios
>   vmscan: Convert page buffer handling to use folios
>   vmscan: Convert lazy freeing to folios
>   vmscan: Move initialisation of mapping down
>   vmscan: Convert the activate_locked portion of shrink_page_list to
>     folios
>   vmscan: Remove remaining uses of page in shrink_page_list
>   mm/shmem: Use a folio in shmem_unused_huge_shrink
>   mm/swap: Add folio_throttle_swaprate
>   mm/shmem: Convert shmem_add_to_page_cache to take a folio
>   mm/shmem: Turn shmem_should_replace_page into
>     shmem_should_replace_folio
>   mm/shmem: Turn shmem_alloc_page() into shmem_alloc_folio()
>   mm/shmem: Convert shmem_alloc_and_acct_page to use a folio
>   mm/shmem: Convert shmem_getpage_gfp to use a folio
>   mm/shmem: Convert shmem_swapin_page() to shmem_swapin_folio()

This series is now in next-20220503 and causes the following clang
warnings:

mm/shmem.c:1704:7: error: variable 'folio' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
                if (!page) {
                    ^~~~~
mm/shmem.c:1761:6: note: uninitialized use occurs here
        if (folio) {
            ^~~~~
mm/shmem.c:1704:3: note: remove the 'if' if its condition is always false
                if (!page) {
                ^~~~~~~~~~~~
mm/shmem.c:1685:21: note: initialize the variable 'folio' to silence this warning
        struct folio *folio;
                           ^
                            = NULL
mm/shmem.c:2340:8: error: variable 'page' is uninitialized when used here [-Werror,-Wuninitialized]
                if (!page)
                     ^~~~
mm/shmem.c:2321:19: note: initialize the variable 'page' to silence this warning
        struct page *page;
                         ^
                          = NULL
2 errors generated.

The first warning is pretty simple as far as I can tell:

diff --git a/mm/shmem.c b/mm/shmem.c
index 820fde6c2ef6..6a18641a90ff 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1682,7 +1682,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 	struct shmem_inode_info *info = SHMEM_I(inode);
 	struct mm_struct *charge_mm = vma ? vma->vm_mm : NULL;
 	struct page *page;
-	struct folio *folio;
+	struct folio *folio = NULL;
 	swp_entry_t swap;
 	int error;
 

However, I am not sure about the second one. It appears to be caused by
patch 18 in this series. Should it have actually been:

diff --git a/mm/shmem.c b/mm/shmem.c
index 820fde6c2ef6..9e0bd0cffe30 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2337,6 +2337,7 @@ int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 
 	if (!*pagep) {
 		ret = -ENOMEM;
+		page = &shmem_alloc_folio(gfp, info, pgoff)->page;
 		if (!page)
 			goto out_unacct_blocks;
 

?

Cheers,
Nathan