Re: [RFC v5][PATCH 7/8] Infrastructure for shared objects

[Serge E. Hallyn]

Quoting Oren Laadan (orenl@xxxxxxxxxxxxxxx):
> Infrastructure to handle objects that may be shared and referenced by
> multiple tasks or other objects, e..g open files, memory address space
> etc.
> 
> The state of shared objects is saved once. On the first encounter, the
> state is dumped and the object is assigned a unique identifier (objref)
> and also stored in a hash table (indexed by its physical kenrel address).
> >From then on the object will be found in the hash and only its identifier
> is saved.
> 
> On restart the identifier is looked up in the hash table; if not found
> then the state is read, the object is created, and added to the hash
> table (this time indexed by its identifier). Otherwise, the object in
> the hash table is used.
> 
> Signed-off-by: Oren Laadan <orenl@xxxxxxxxxxxxxxx>

Acked-by: Serge Hallyn <serue@xxxxxxxxxx>

Thanks, Oren, I actually think this is quite nice and readable.

Though three questions below.  First one is, since you've mentioned
having multiple threads doing checkpoint, won't you need some locking?
I assume that's coming in later patches if/when needed?

> ---
>  Documentation/checkpoint.txt |   38 +++++++
>  checkpoint/Makefile          |    2 +-
>  checkpoint/objhash.c         |  237 ++++++++++++++++++++++++++++++++++++++++++
>  checkpoint/sys.c             |    4 +
>  include/linux/checkpoint.h   |   20 ++++
>  5 files changed, 300 insertions(+), 1 deletions(-)
>  create mode 100644 checkpoint/objhash.c
> 
> diff --git a/Documentation/checkpoint.txt b/Documentation/checkpoint.txt
> index 6bf75ce..2929512 100644
> --- a/Documentation/checkpoint.txt
> +++ b/Documentation/checkpoint.txt
> @@ -169,6 +169,44 @@ cr_hdr + cr_hdr_task
>  cr_hdr + cr_hdr_tail
> 
> 
> +=== Shared resources (objects)
> +
> +Many resources used by tasks may be shared by more than one task (e.g.
> +file descriptors, memory address space, etc), or even have multiple
> +references from other resources (e.g. a single inode that represents
> +two ends of a pipe).
> +
> +Clearly, the state of shared objects need only be saved once, even if
> +they occur multiple times. We use a hash table (ctx->objhash) to keep
> +track of shared objects in the following manner.
> +
> +On the first encounter, the state is dumped and the object is assigned
> +a unique identifier and also stored in the hash table (indexed by its
> +physical kenrel address). From then on the object will be found in the
> +hash and only its identifier is saved.
> +
> +On restart the identifier is looked up in the hash table; if not found
> +then the state is read, the object is created, and added to the hash
> +table (this time indexed by its identifier). Otherwise, the object in
> +the hash table is used.
> +
> +The interface for the hash table is the following:
> +
> +cr_obj_get_by_ptr - find the unique identifier - object reference (objref)
> +  of the object that is pointer to by ptr (or 0 if not found) [checkpoint]
> +
> +cr_obj_add_ptr - add the object pointed to by ptr to the hash table if
> +  it isn't already there, and fill its unique identifier (objref); will
> +  return 0 if already found in the has, or 1 otherwise [checkpoint]
> +
> +cr_obj_get_by_ref - return the pointer to the object whose unique identifier
> +  is equal to objref [restart]
> +
> +cr_obj_add_ref - add the object with unique identifier objref, pointed to by
> +  ptr to the hash table if it isn't already there; will return 0 if already
> +  found in the has, or 1 otherwise [restart]
> +
> +
>  === Changelog
> 
>  [2008-Sep-11] v5:
> diff --git a/checkpoint/Makefile b/checkpoint/Makefile
> index ac35033..9843fb9 100644
> --- a/checkpoint/Makefile
> +++ b/checkpoint/Makefile
> @@ -2,5 +2,5 @@
>  # Makefile for linux checkpoint/restart.
>  #
> 
> -obj-$(CONFIG_CHECKPOINT_RESTART) += sys.o checkpoint.o restart.o \
> +obj-$(CONFIG_CHECKPOINT_RESTART) += sys.o checkpoint.o restart.o objhash.o \
>  		ckpt_mem.o rstr_mem.o
> diff --git a/checkpoint/objhash.c b/checkpoint/objhash.c
> new file mode 100644
> index 0000000..0862086
> --- /dev/null
> +++ b/checkpoint/objhash.c
> @@ -0,0 +1,237 @@
> +/*
> + *  Checkpoint-restart - object hash infrastructure to manage shared objects
> + *
> + *  Copyright (C) 2008 Oren Laadan
> + *
> + *  This file is subject to the terms and conditions of the GNU General Public
> + *  License.  See the file COPYING in the main directory of the Linux
> + *  distribution for more details.
> + */
> +
> +#include <linux/kernel.h>
> +#include <linux/file.h>
> +#include <linux/hash.h>
> +#include <linux/checkpoint.h>
> +
> +struct cr_objref {
> +	int objref;
> +	void *ptr;
> +	unsigned short type;

What is the point of the 'type'?

By that I mean: is it meant to catch bugs in the implementation, or bad
checkpoint images?

> +	unsigned short flags;
> +	struct hlist_node hash;
> +};
> +
> +struct cr_objhash {
> +	struct hlist_head *head;
> +	int objref_index;

What exactly will objref_index be used for?  I don't see any real
usage here or in your later patches.

> +};
> +
> +#define CR_OBJHASH_NBITS  10
> +#define CR_OBJHASH_TOTAL  (1UL << CR_OBJHASH_NBITS)
> +
> +static void cr_obj_ref_drop(struct cr_objref *obj)
> +{
> +	switch (obj->type) {
> +	case CR_OBJ_FILE:
> +		fput((struct file *) obj->ptr);
> +		break;
> +	default:
> +		BUG();
> +	}
> +}
> +
> +static void cr_obj_ref_grab(struct cr_objref *obj)
> +{
> +	switch (obj->type) {
> +	case CR_OBJ_FILE:
> +		get_file((struct file *) obj->ptr);
> +		break;
> +	default:
> +		BUG();
> +	}
> +}
> +
> +static void cr_objhash_clear(struct cr_objhash *objhash)
> +{
> +	struct hlist_head *h = objhash->head;
> +	struct hlist_node *n, *t;
> +	struct cr_objref *obj;
> +	int i;
> +
> +	for (i = 0; i < CR_OBJHASH_TOTAL; i++) {
> +		hlist_for_each_entry_safe(obj, n, t, &h[i], hash) {
> +			cr_obj_ref_drop(obj);
> +			kfree(obj);
> +		}
> +	}
> +}
> +
> +void cr_objhash_free(struct cr_ctx *ctx)
> +{
> +	struct cr_objhash *objhash = ctx->objhash;
> +
> +	if (objhash) {
> +		cr_objhash_clear(objhash);
> +		kfree(objhash->head);
> +		kfree(ctx->objhash);
> +		ctx->objhash = NULL;
> +	}
> +}
> +
> +int cr_objhash_alloc(struct cr_ctx *ctx)
> +{
> +	struct cr_objhash *objhash;
> +	struct hlist_head *head;
> +
> +	objhash = kzalloc(sizeof(*objhash), GFP_KERNEL);
> +	if (!objhash)
> +		return -ENOMEM;
> +	head = kzalloc(CR_OBJHASH_TOTAL * sizeof(*head), GFP_KERNEL);
> +	if (!head) {
> +		kfree(objhash);
> +		return -ENOMEM;
> +	}
> +
> +	objhash->head = head;
> +	objhash->objref_index = 1;
> +
> +	ctx->objhash = objhash;
> +	return 0;
> +}
> +
> +static struct cr_objref *cr_obj_find_by_ptr(struct cr_ctx *ctx, void *ptr)
> +{
> +	struct hlist_head *h;
> +	struct hlist_node *n;
> +	struct cr_objref *obj;
> +
> +	h = &ctx->objhash->head[hash_ptr(ptr, CR_OBJHASH_NBITS)];
> +	hlist_for_each_entry(obj, n, h, hash)
> +		if (obj->ptr == ptr)
> +			return obj;
> +	return NULL;
> +}
> +
> +static struct cr_objref *cr_obj_find_by_objref(struct cr_ctx *ctx, int objref)
> +{
> +	struct hlist_head *h;
> +	struct hlist_node *n;
> +	struct cr_objref *obj;
> +
> +	h = &ctx->objhash->head[hash_ptr((void *) objref, CR_OBJHASH_NBITS)];
> +	hlist_for_each_entry(obj, n, h, hash)
> +		if (obj->objref == objref)
> +			return obj;
> +	return NULL;
> +}
> +
> +static struct cr_objref *cr_obj_new(struct cr_ctx *ctx, void *ptr, int objref,
> +				    unsigned short type, unsigned short flags)
> +{
> +	struct cr_objref *obj;
> +
> +	obj = kmalloc(sizeof(*obj), GFP_KERNEL);
> +	if (obj) {
> +		int i;
> +
> +		obj->ptr = ptr;
> +		obj->type = type;
> +		obj->flags = flags;
> +
> +		if (objref) {
> +			/* use 'objref' to index (restart) */
> +			obj->objref = objref;
> +			i = hash_ptr((void *) objref, CR_OBJHASH_NBITS);
> +		} else {
> +			/* use 'ptr' to index, assign objref (checkpoint) */
> +			obj->objref = ctx->objhash->objref_index++;;
> +			i = hash_ptr(ptr, CR_OBJHASH_NBITS);
> +		}
> +
> +		hlist_add_head(&obj->hash, &ctx->objhash->head[i]);
> +		cr_obj_ref_grab(obj);
> +	}
> +	return obj;
> +}
> +
> +/**
> + * cr_obj_add_ptr - add an object to the hash table if not already there
> + * @ctx: checkpoint context
> + * @ptr: pointer to object
> + * @objref: unique identifier - object reference [output]
> + * @type: object type
> + * @flags: object flags
> + *
> + * Fills the unique identifier of the object into @objref
> + *
> + * returns 0 if found, 1 if added, < 0 on error
> + */
> +int cr_obj_add_ptr(struct cr_ctx *ctx, void *ptr, int *objref,
> +		   unsigned short type, unsigned short flags)
> +{
> +	struct cr_objref *obj;
> +	int ret = 0;
> +
> +	obj = cr_obj_find_by_ptr(ctx, ptr);
> +	if (!obj) {
> +		obj = cr_obj_new(ctx, ptr, 0, type, flags);
> +		if (!obj)
> +			return -ENOMEM;
> +		else
> +			ret = 1;
> +	} else if (obj->type != type)	/* sanity check */
> +		return -EINVAL;
> +	*objref = obj->objref;
> +	return ret;
> +}
> +
> +/**
> + * cr_obj_add_ref - add an object with unique identifer to the hash table
> + * @ctx: checkpoint context
> + * @ptr: pointer to object
> + * @objref: unique identifier - object reference
> + * @type: object type
> + * @flags: object flags
> + */
> +int cr_obj_add_ref(struct cr_ctx *ctx, void *ptr, int objref,
> +		   unsigned short type, unsigned short flags)
> +{
> +	struct cr_objref *obj;
> +
> +	obj = cr_obj_new(ctx, ptr, objref, type, flags);
> +	return obj ? 0 : -ENOMEM;
> +}
> +
> +/**
> + * cr_obj_get_by_ptr - find the unique identifier (objref) of an object
> + * @ctx: checkpoint context
> + * @ptr: pointer to object
> + * @type: object type
> + */
> +int cr_obj_get_by_ptr(struct cr_ctx *ctx, void *ptr, unsigned short type)
> +{
> +	struct cr_objref *obj;
> +
> +	obj = cr_obj_find_by_ptr(ctx, ptr);
> +	if (obj)
> +		return obj->type == type ? obj->objref : -EINVAL;
> +	else
> +		return -ESRCH;
> +}
> +
> +/**
> + * cr_obj_get_by_ref - find an object given its unique identifier (objref)
> + * @ctx: checkpoint context
> + * @objref: unique identifier - object reference
> + * @type: object type
> + */
> +void *cr_obj_get_by_ref(struct cr_ctx *ctx, int objref, unsigned short type)
> +{
> +	struct cr_objref *obj;
> +
> +	obj = cr_obj_find_by_objref(ctx, objref);
> +	if (obj)
> +		return obj->type == type ? obj->ptr : ERR_PTR(-EINVAL);
> +	else
> +		return NULL;
> +}
> diff --git a/checkpoint/sys.c b/checkpoint/sys.c
> index c4ac157..3b17ecc 100644
> --- a/checkpoint/sys.c
> +++ b/checkpoint/sys.c
> @@ -132,6 +132,7 @@ void cr_ctx_free(struct cr_ctx *ctx)
>  		path_put(ctx->vfsroot);
> 
>  	cr_pgarr_free(ctx);
> +	cr_objhash_free(ctx);
> 
>  	kfree(ctx);
>  }
> @@ -154,6 +155,9 @@ struct cr_ctx *cr_ctx_alloc(pid_t pid, int fd, unsigned long flags)
>  	if (!ctx->hbuf)
>  		goto nomem;
> 
> +	if (cr_objhash_alloc(ctx) < 0)
> +		goto nomem;
> +
>  	/*
>  	 * assume checkpointer is in container's root vfs
>  	 * FIXME: this works for now, but will change with real containers
> diff --git a/include/linux/checkpoint.h b/include/linux/checkpoint.h
> index a9eafac..9990bb1 100644
> --- a/include/linux/checkpoint.h
> +++ b/include/linux/checkpoint.h
> @@ -28,6 +28,8 @@ struct cr_ctx {
>  	void *hbuf;		/* temporary buffer for headers */
>  	int hpos;		/* position in headers buffer */
> 
> +	struct cr_objhash *objhash;	/* hash for shared objects */
> +
>  	struct list_head pgarr_list;	/* page array to dump VMA contents */
> 
>  	struct path *vfsroot;	/* container root (FIXME) */
> @@ -49,6 +51,24 @@ extern int cr_kread(struct cr_ctx *ctx, void *buf, int count);
>  extern void *cr_hbuf_get(struct cr_ctx *ctx, int n);
>  extern void cr_hbuf_put(struct cr_ctx *ctx, int n);
> 
> +/* shared objects handling */
> +
> +enum {
> +	CR_OBJ_FILE = 1,
> +	CR_OBJ_MAX
> +};
> +
> +extern void cr_objhash_free(struct cr_ctx *ctx);
> +extern int cr_objhash_alloc(struct cr_ctx *ctx);
> +extern void *cr_obj_get_by_ref(struct cr_ctx *ctx,
> +			       int objref, unsigned short type);
> +extern int cr_obj_get_by_ptr(struct cr_ctx *ctx,
> +			     void *ptr, unsigned short type);
> +extern int cr_obj_add_ptr(struct cr_ctx *ctx, void *ptr, int *objref,
> +			  unsigned short type, unsigned short flags);
> +extern int cr_obj_add_ref(struct cr_ctx *ctx, void *ptr, int objref,
> +			  unsigned short type, unsigned short flags);
> +
>  struct cr_hdr;
> 
>  extern int cr_write_obj(struct cr_ctx *ctx, struct cr_hdr *h, void *buf);
> -- 
> 1.5.4.3
> 
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