Re: [PATCH v3 4/6] mm: Introduce a pageflag for partially mapped folios

[Usama Arif <usamaarif642@gmail.com>]

On 16/08/2024 16:44, Matthew Wilcox wrote:
> On Tue, Aug 13, 2024 at 01:02:47PM +0100, Usama Arif wrote:
>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>> index a0a29bd092f8..cecc1bad7910 100644
>> --- a/include/linux/page-flags.h
>> +++ b/include/linux/page-flags.h
>> @@ -182,6 +182,7 @@ enum pageflags {
>>  	/* At least one page in this folio has the hwpoison flag set */
>>  	PG_has_hwpoisoned = PG_active,
>>  	PG_large_rmappable = PG_workingset, /* anon or file-backed */
>> +	PG_partially_mapped, /* was identified to be partially mapped */
> 
> No, you can't do this.  You have to be really careful when reusing page
> flags, you can't just take the next one.  What made you think it would
> be this easy?
> 
> I'd suggest using PG_reclaim.  You also need to add PG_partially_mapped
> to PAGE_FLAGS_SECOND.  You might get away without that if you're
> guaranteeing it'll always be clear when you free the folio; I don't
> understand this series so I don't know if that's true or not.

I am really not sure what the issue is over here.

From what I see, bits 0-7 of folio->_flags_1 are used for storing folio order, bit 8 for PG_has_hwpoisoned and bit 9 for PG_large_rmappable.
Bits 10 and above of folio->_flags_1 are not used any anywhere in the kernel. I am not reusing a page flag of folio->_flags_1, just taking an unused one.

Please have a look at the next few lines of the patch. I have defined the functions as FOLIO_FLAG(partially_mapped, FOLIO_SECOND_PAGE). I believe thats what you are saying in your second paragraph?
I am not sure what you meant by using PG_reclaim.

I have added the next few lines of the patch below:

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index a0a29bd092f8..cecc1bad7910 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -182,6 +182,7 @@ enum pageflags {
 	/* At least one page in this folio has the hwpoison flag set */
 	PG_has_hwpoisoned = PG_active,
 	PG_large_rmappable = PG_workingset, /* anon or file-backed */
+	PG_partially_mapped, /* was identified to be partially mapped */
 };
 
 #define PAGEFLAGS_MASK		((1UL << NR_PAGEFLAGS) - 1)
@@ -861,8 +862,10 @@ static inline void ClearPageCompound(struct page *page)
 	ClearPageHead(page);
 }
 FOLIO_FLAG(large_rmappable, FOLIO_SECOND_PAGE)
+FOLIO_FLAG(partially_mapped, FOLIO_SECOND_PAGE)
 #else
 FOLIO_FLAG_FALSE(large_rmappable)