linux-kernel - Re: [PATCH v2] lib: Replace kmap() with kmap_local

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Message-ID: <20230619164035.GC412243@sumitra.com>
Date:   Mon, 19 Jun 2023 09:40:35 -0700
From:   Sumitra Sharma <sumitraartsy@...il.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Jérôme Glisse <jglisse@...hat.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Ira Weiny <ira.weiny@...el.com>,
        Deepak R Varma <drv@...lo.com>,
        Sumitra Sharma <sumitraartsy@...il.com>
Subject: Re: [PATCH v2] lib: Replace kmap() with kmap_local_page()

On Sun, Jun 18, 2023 at 06:50:04AM +0200, Fabio M. De Francesco wrote:
> On sabato 10 giugno 2023 19:57:12 CEST Sumitra Sharma wrote:
> > kmap() has been deprecated in favor of the kmap_local_page()
> > due to high cost, restricted mapping space, the overhead of
> > a global lock for synchronization, and making the process
> > sleep in the absence of free slots.
> > 
> > kmap_local_page() is faster than kmap() and offers thread-local
> > and CPU-local mappings, take pagefaults in a local kmap region
> > and preserves preemption by saving the mappings of outgoing
> > tasks and restoring those of the incoming one during a context
> > switch.
> > 
> > The mappings are kept thread local in the functions
> > “dmirror_do_read” and “dmirror_do_write” in test_hmm.c
> > 
> > Therefore, replace kmap() with kmap_local_page() and use
> > mempcy_from/to_page() to avoid open coding kmap_local_page()
> > + memcpy() + kunmap_local().
> > 
> > Remove the unused variable “tmp”.
> > 
> > 
> > Suggested-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> 
> LGTM, so...
> 
> Reviewed-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> 
> Thanks,
> 
> Fabio
> 
> P.S.: The answer to an old question from you is that "LGTM" stands for "[It] 
> Looks Good To Me". 
> 
> It's just a way to introduce the "Reviewed-by" tag itself. However, "LGTM" is 
> not required, whereas the tag is required for a valid review and only the tag 
> line will be added by maintainers in the log when your patch gets applied.  
> 
> While here... Please don't put unnecessary blank lines between the tags.They 
> are not required and instead may worsen readability (obviously, I'm not 
> requiring a new version for this).  
> 

Hi Fabio,

Thank you for your explanation and clarification regarding the meaning of "LGTM." 
I will take care of the blank lines.


Thanks & regards
Sumitra

> > 
> > Signed-off-by: Sumitra Sharma <sumitraartsy@...il.com>
> > ---
> > 
> > Changes in v2:
> > 	- Change commit subject and description.
> > 	- Remove unnecessary type casting (char *).
> > 
> > 
> >  lib/test_hmm.c | 10 ++--------
> >  1 file changed, 2 insertions(+), 8 deletions(-)
> > 
> > diff --git a/lib/test_hmm.c b/lib/test_hmm.c
> > index 67e6f83fe0f8..717dcb830127 100644
> > --- a/lib/test_hmm.c
> > +++ b/lib/test_hmm.c
> > @@ -368,16 +368,13 @@ static int dmirror_do_read(struct dmirror *dmirror,
> > unsigned long start, for (pfn = start >> PAGE_SHIFT; pfn < (end >>
> > PAGE_SHIFT); pfn++) { void *entry;
> >  		struct page *page;
> > -		void *tmp;
> > 
> >  		entry = xa_load(&dmirror->pt, pfn);
> >  		page = xa_untag_pointer(entry);
> >  		if (!page)
> >  			return -ENOENT;
> > 
> > -		tmp = kmap(page);
> > -		memcpy(ptr, tmp, PAGE_SIZE);
> > -		kunmap(page);
> > +		memcpy_from_page(ptr, page, 0, PAGE_SIZE);
> > 
> >  		ptr += PAGE_SIZE;
> >  		bounce->cpages++;
> > @@ -437,16 +434,13 @@ static int dmirror_do_write(struct dmirror *dmirror,
> > unsigned long start, for (pfn = start >> PAGE_SHIFT; pfn < (end >>
> > PAGE_SHIFT); pfn++) { void *entry;
> >  		struct page *page;
> > -		void *tmp;
> > 
> >  		entry = xa_load(&dmirror->pt, pfn);
> >  		page = xa_untag_pointer(entry);
> >  		if (!page || xa_pointer_tag(entry) != DPT_XA_TAG_WRITE)
> >  			return -ENOENT;
> > 
> > -		tmp = kmap(page);
> > -		memcpy(tmp, ptr, PAGE_SIZE);
> > -		kunmap(page);
> > +		memcpy_to_page(page, 0, ptr, PAGE_SIZE);
> > 
> >  		ptr += PAGE_SIZE;
> >  		bounce->cpages++;
> > --
> > 2.25.1
> 
> 
> 
>