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SubjectRe: [PATCH] vmalloc: respect the GFP_NOIO and GFP_NOFS flags
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On 06/29/2017 07:25 PM, Mikulas Patocka wrote:
> The __vmalloc function has a parameter gfp_mask with the allocation flags,
> however it doesn't fully respect the GFP_NOIO and GFP_NOFS flags. The
> pages are allocated with the specified gfp flags, but the pagetables are
> always allocated with GFP_KERNEL. This allocation can cause unexpected
> recursion into the filesystem or I/O subsystem.
>
> It is not practical to extend page table allocation routines with gfp
> flags because it would require modification of architecture-specific code
> in all architecturs. However, the process can temporarily request that all
> allocations are done with GFP_NOFS or GFP_NOIO with with the functions
> memalloc_nofs_save and memalloc_noio_save.
>
> This patch makes the vmalloc code use memalloc_nofs_save or
> memalloc_noio_save if the supplied gfp flags do not contain __GFP_FS or
> __GFP_IO. It fixes some possible deadlocks in drivers/mtd/ubi/io.c,
> fs/gfs2/, fs/btrfs/free-space-tree.c, fs/ubifs/,
> fs/nfs/blocklayout/extent_tree.c where __vmalloc is used with the GFP_NOFS
> flag.
>
> The patch also simplifies code in dm-bufio.c, dm-ioctl.c and fs/xfs/kmem.c
> by removing explicit calls to memalloc_nofs_save and memalloc_noio_save
> before the call to __vmalloc.
>
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
>
> ---
> drivers/md/dm-bufio.c | 24 +-----------------------
> drivers/md/dm-ioctl.c | 6 +-----
> fs/xfs/kmem.c | 14 --------------
> mm/util.c | 6 +++---
> mm/vmalloc.c | 18 +++++++++++++++++-
> 5 files changed, 22 insertions(+), 46 deletions(-)
>
> Index: linux-2.6/mm/vmalloc.c
> ===================================================================
> --- linux-2.6.orig/mm/vmalloc.c
> +++ linux-2.6/mm/vmalloc.c
> @@ -31,6 +31,7 @@
> #include <linux/compiler.h>
> #include <linux/llist.h>
> #include <linux/bitops.h>
> +#include <linux/sched/mm.h>
>
> #include <linux/uaccess.h>
> #include <asm/tlbflush.h>
> @@ -1670,6 +1671,8 @@ static void *__vmalloc_area_node(struct
> unsigned int nr_pages, array_size, i;
> const gfp_t nested_gfp = (gfp_mask & GFP_RECLAIM_MASK) | __GFP_ZERO;
> const gfp_t alloc_mask = gfp_mask | __GFP_HIGHMEM | __GFP_NOWARN;
> + unsigned noio_flag;
> + int r;
>
> nr_pages = get_vm_area_size(area) >> PAGE_SHIFT;
> array_size = (nr_pages * sizeof(struct page *));
> @@ -1712,8 +1715,21 @@ static void *__vmalloc_area_node(struct
> cond_resched();
> }
>
> - if (map_vm_area(area, prot, pages))
> + if (unlikely(!(gfp_mask & __GFP_IO)))
> + noio_flag = memalloc_noio_save();
> + else if (unlikely(!(gfp_mask & __GFP_FS)))
> + noio_flag = memalloc_nofs_save();
> +
> + r = map_vm_area(area, prot, pages);
> +
> + if (unlikely(!(gfp_mask & __GFP_IO)))
> + memalloc_noio_restore(noio_flag);
> + else if (unlikely(!(gfp_mask & __GFP_FS)))
> + memalloc_nofs_restore(noio_flag);
> +
> + if (unlikely(r))
> goto fail;
> +
> return area->addr;
>
> fail:
> Index: linux-2.6/mm/util.c
> ===================================================================
> --- linux-2.6.orig/mm/util.c
> +++ linux-2.6/mm/util.c
> @@ -351,10 +351,10 @@ void *kvmalloc_node(size_t size, gfp_t f
> void *ret;
>
> /*
> - * vmalloc uses GFP_KERNEL for some internal allocations (e.g page tables)
> - * so the given set of flags has to be compatible.
> + * vmalloc uses blocking allocations for some internal allocations
> + * (e.g page tables) so the given set of flags has to be compatible.
> */
> - WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL);
> + WARN_ON_ONCE(!gfpflags_allow_blocking(flags));

Hi Mikulas,

OK, so given the new behavior in the underlying __vmalloc code, I think it's
appropriate to add this documentation change on top of what you have so far:

mm/util.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/mm/util.c b/mm/util.c
index cdbc9022c021..39fe94530dd2 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -343,7 +343,8 @@ EXPORT_SYMBOL(vm_mmap);
* __GFP_RETRY_MAYFAIL is supported, and it should be used only if kmalloc is
* preferable to the vmalloc fallback, due to visible performance drawbacks.
*
- * Any use of gfp flags outside of GFP_KERNEL should be consulted with mm people.
+ * Any use of gfp flags other than GFP_KERNEL, GFP_NOIO, or GFP_NOFS should
+ * be done only after consulting with mm people.
*/
void *kvmalloc_node(size_t size, gfp_t flags, int node)
{
>
> /*
> * We want to attempt a large physically contiguous block first because
> Index: linux-2.6/drivers/md/dm-bufio.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm-bufio.c
> +++ linux-2.6/drivers/md/dm-bufio.c
> @@ -386,9 +386,6 @@ static void __cache_size_refresh(void)
> static void *alloc_buffer_data(struct dm_bufio_client *c, gfp_t gfp_mask,
> enum data_mode *data_mode)
> {
> - unsigned noio_flag;
> - void *ptr;
> -
> if (c->block_size <= DM_BUFIO_BLOCK_SIZE_SLAB_LIMIT) {
> *data_mode = DATA_MODE_SLAB;
> return kmem_cache_alloc(DM_BUFIO_CACHE(c), gfp_mask);
> @@ -402,26 +399,7 @@ static void *alloc_buffer_data(struct dm
> }
>
> *data_mode = DATA_MODE_VMALLOC;
> -
> - /*
> - * __vmalloc allocates the data pages and auxiliary structures with
> - * gfp_flags that were specified, but pagetables are always allocated
> - * with GFP_KERNEL, no matter what was specified as gfp_mask.
> - *
> - * Consequently, we must set per-process flag PF_MEMALLOC_NOIO so that
> - * all allocations done by this process (including pagetables) are done
> - * as if GFP_NOIO was specified.
> - */
> -
> - if (gfp_mask & __GFP_NORETRY)
> - noio_flag = memalloc_noio_save();
> -
> - ptr = __vmalloc(c->block_size, gfp_mask, PAGE_KERNEL);
> -
> - if (gfp_mask & __GFP_NORETRY)
> - memalloc_noio_restore(noio_flag);
> -
> - return ptr;
> + return __vmalloc(c->block_size, gfp_mask, PAGE_KERNEL);

Seems like a nice cleanup, and although I'm not an expert, it looks
correct to me.

> }
>
> /*
> Index: linux-2.6/drivers/md/dm-ioctl.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm-ioctl.c
> +++ linux-2.6/drivers/md/dm-ioctl.c
> @@ -1691,7 +1691,6 @@ static int copy_params(struct dm_ioctl _
> struct dm_ioctl *dmi;
> int secure_data;
> const size_t minimum_data_size = offsetof(struct dm_ioctl, data);
> - unsigned noio_flag;
>
> if (copy_from_user(param_kernel, user, minimum_data_size))
> return -EFAULT;
> @@ -1714,10 +1713,7 @@ static int copy_params(struct dm_ioctl _
> * suspended and the ioctl is needed to resume it.
> * Use kmalloc() rather than vmalloc() when we can.
> */
> - dmi = NULL;
> - noio_flag = memalloc_noio_save();
> - dmi = kvmalloc(param_kernel->data_size, GFP_KERNEL | __GFP_HIGH);
> - memalloc_noio_restore(noio_flag);
> + dmi = kvmalloc(param_kernel->data_size, GFP_NOIO | __GFP_HIGH);

Pre-existing, and apologies if I missed a conversation about it, but is
__GFP_HIGH necessary and appropriate for kvmalloc()? If so, maybe we
should add it to the list of approved GFP flags in the function's
documentation. But I tentatively think you can just pass in GFP_NOIO
instead, and get the right behavior.

thanks,
john h

>
> if (!dmi) {
> if (secure_data && clear_user(user, param_kernel->data_size))
> Index: linux-2.6/fs/xfs/kmem.c
> ===================================================================
> --- linux-2.6.orig/fs/xfs/kmem.c
> +++ linux-2.6/fs/xfs/kmem.c
> @@ -48,7 +48,6 @@ kmem_alloc(size_t size, xfs_km_flags_t f
> void *
> kmem_zalloc_large(size_t size, xfs_km_flags_t flags)
> {
> - unsigned nofs_flag = 0;
> void *ptr;
> gfp_t lflags;
>
> @@ -56,22 +55,9 @@ kmem_zalloc_large(size_t size, xfs_km_fl
> if (ptr)
> return ptr;
>
> - /*
> - * __vmalloc() will allocate data pages and auxillary structures (e.g.
> - * pagetables) with GFP_KERNEL, yet we may be under GFP_NOFS context
> - * here. Hence we need to tell memory reclaim that we are in such a
> - * context via PF_MEMALLOC_NOFS to prevent memory reclaim re-entering
> - * the filesystem here and potentially deadlocking.
> - */
> - if (flags & KM_NOFS)
> - nofs_flag = memalloc_nofs_save();
> -
> lflags = kmem_flags_convert(flags);
> ptr = __vmalloc(size, lflags | __GFP_ZERO, PAGE_KERNEL);
>
> - if (flags & KM_NOFS)
> - memalloc_nofs_restore(nofs_flag);
> -
> return ptr;
> }
>
>

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