Re: [PATCH v2] intel_th: avoid using deprecated page->mapping, index fields

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

Hi Alexander,

Happy New Year! I realise it's been the holidays recently, but Wondering if
you'd had a chance to look at this?

Thanks, Lorenzo

On Tue, Dec 03, 2024 at 08:00:01AM +0000, Lorenzo Stoakes wrote:
> The struct page->mapping, index fields are deprecated and soon to be only
> available as part of a folio.
>
> It is likely the intel_th code which sets page->mapping, index is was
> implemented out of concern that some aspect of the page fault logic may
> encounter unexpected problems should they not.
>
> However, the appropriate interface for inserting kernel-allocated memory is
> vm_insert_page() in a VM_MIXEDMAP. By using the helper function
> vmf_insert_mixed() we can do this with minimal churn in the existing fault
> handler.
>
> By doing so, we bypass the remainder of the faulting logic. The pages are
> still pinned so there is no possibility of anything unexpected being done
> with the pages once established.
>
> It would also be reasonable to pre-map everything on fault, however to
> minimise churn we retain the fault handler.
>
> We also eliminate all code which clears page->mapping on teardown as this
> has now become unnecessary.
>
> The MSU code relies on faulting to function correctly, so is by definition
> dependent on CONFIG_MMU. We avoid spurious reports about compilation
> failure for unsupported platforms by making this requirement explicit in
> Kconfig as part of this change too.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> ---
>
> v2:
> * Make the MSU driver dependent on CONFIG_MMU to avoid spurious compilation
>   failure reports.
>
> v1:
> https://lore.kernel.org/all/20241202122127.51313-1-lorenzo.stoakes@oracle.com/
>
>  drivers/hwtracing/intel_th/Kconfig |  1 +
>  drivers/hwtracing/intel_th/msu.c   | 31 +++++++-----------------------
>  2 files changed, 8 insertions(+), 24 deletions(-)
>
> diff --git a/drivers/hwtracing/intel_th/Kconfig b/drivers/hwtracing/intel_th/Kconfig
> index 4b6359326ede..4f7d2b6d79e2 100644
> --- a/drivers/hwtracing/intel_th/Kconfig
> +++ b/drivers/hwtracing/intel_th/Kconfig
> @@ -60,6 +60,7 @@ config INTEL_TH_STH
>
>  config INTEL_TH_MSU
>  	tristate "Intel(R) Trace Hub Memory Storage Unit"
> +	depends on MMU
>  	help
>  	  Memory Storage Unit (MSU) trace output device enables
>  	  storing STP traces to system memory. It supports single
> diff --git a/drivers/hwtracing/intel_th/msu.c b/drivers/hwtracing/intel_th/msu.c
> index 66123d684ac9..93b65a9731d7 100644
> --- a/drivers/hwtracing/intel_th/msu.c
> +++ b/drivers/hwtracing/intel_th/msu.c
> @@ -19,6 +19,7 @@
>  #include <linux/io.h>
>  #include <linux/workqueue.h>
>  #include <linux/dma-mapping.h>
> +#include <linux/pfn_t.h>
>
>  #ifdef CONFIG_X86
>  #include <asm/set_memory.h>
> @@ -967,7 +968,6 @@ static void msc_buffer_contig_free(struct msc *msc)
>  	for (off = 0; off < msc->nr_pages << PAGE_SHIFT; off += PAGE_SIZE) {
>  		struct page *page = virt_to_page(msc->base + off);
>
> -		page->mapping = NULL;
>  		__free_page(page);
>  	}
>
> @@ -1149,9 +1149,6 @@ static void __msc_buffer_win_free(struct msc *msc, struct msc_window *win)
>  	int i;
>
>  	for_each_sg(win->sgt->sgl, sg, win->nr_segs, i) {
> -		struct page *page = msc_sg_page(sg);
> -
> -		page->mapping = NULL;
>  		dma_free_coherent(msc_dev(win->msc)->parent->parent, PAGE_SIZE,
>  				  sg_virt(sg), sg_dma_address(sg));
>  	}
> @@ -1592,22 +1589,10 @@ static void msc_mmap_close(struct vm_area_struct *vma)
>  {
>  	struct msc_iter *iter = vma->vm_file->private_data;
>  	struct msc *msc = iter->msc;
> -	unsigned long pg;
>
>  	if (!atomic_dec_and_mutex_lock(&msc->mmap_count, &msc->buf_mutex))
>  		return;
>
> -	/* drop page _refcounts */
> -	for (pg = 0; pg < msc->nr_pages; pg++) {
> -		struct page *page = msc_buffer_get_page(msc, pg);
> -
> -		if (WARN_ON_ONCE(!page))
> -			continue;
> -
> -		if (page->mapping)
> -			page->mapping = NULL;
> -	}
> -
>  	/* last mapping -- drop user_count */
>  	atomic_dec(&msc->user_count);
>  	mutex_unlock(&msc->buf_mutex);
> @@ -1617,16 +1602,14 @@ static vm_fault_t msc_mmap_fault(struct vm_fault *vmf)
>  {
>  	struct msc_iter *iter = vmf->vma->vm_file->private_data;
>  	struct msc *msc = iter->msc;
> +	struct page *page;
>
> -	vmf->page = msc_buffer_get_page(msc, vmf->pgoff);
> -	if (!vmf->page)
> +	page = msc_buffer_get_page(msc, vmf->pgoff);
> +	if (!page)
>  		return VM_FAULT_SIGBUS;
>
> -	get_page(vmf->page);
> -	vmf->page->mapping = vmf->vma->vm_file->f_mapping;
> -	vmf->page->index = vmf->pgoff;
> -
> -	return 0;
> +	get_page(page);
> +	return vmf_insert_mixed(vmf->vma, vmf->address, page_to_pfn_t(page));
>  }
>
>  static const struct vm_operations_struct msc_mmap_ops = {
> @@ -1667,7 +1650,7 @@ static int intel_th_msc_mmap(struct file *file, struct vm_area_struct *vma)
>  		atomic_dec(&msc->user_count);
>
>  	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> -	vm_flags_set(vma, VM_DONTEXPAND | VM_DONTCOPY);
> +	vm_flags_set(vma, VM_DONTEXPAND | VM_DONTCOPY | VM_MIXEDMAP);
>  	vma->vm_ops = &msc_mmap_ops;
>  	return ret;
>  }
> --
> 2.47.1