linux-kernel - Re: [PATCH] iov_iter: call kmap on each page even for lowmem if CONFIG_DEBUG_KMAP_LOCAL_FORCE

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Date: Sat, 2 Mar 2024 12:32:07 +0800
From: Herbert Xu <herbert@...dor.apana.org.au>
To: Barry Song <21cnbao@...il.com>
Cc: viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH] iov_iter: call kmap on each page even for lowmem if
 CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP is enabled

On Sat, Mar 02, 2024 at 12:09:08PM +1300, Barry Song wrote:
> From: Barry Song <v-songbaohua@...o.com>
> 
> copy_page_from_iter_atomic() has the assumption lowmem will only
> need one kmap to get start page_address() for all pages. This is
> wrong if the debug option CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP is
> enabled. This patch fixes it in the same way with skbuff.h by
> always applying kmap one by one even for lowmem,
> 
>  static inline bool skb_frag_must_loop(struct page *p)
>  {
>  #if defined(CONFIG_HIGHMEM)
>  	if (IS_ENABLED(CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP) || PageHighMem(p))
>   		return true;
>  #endif
>  	return false;
>  }

Thanks for the patch.  Perhaps this could be moved into highmem.h
as a helper (kmap_is_highmem)?
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt