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* [PATCH] cache: Tolerate short writes in print_slot
@ 2021-09-10 14:18 Hristo Venev
  2021-10-07  9:35 ` John Keeping
  0 siblings, 1 reply; 3+ messages in thread
From: Hristo Venev @ 2021-09-10 14:18 UTC (permalink / raw)
  To: cgit; +Cc: Hristo Venev

sendfile() can return after a short read/write, so we may need to call
it more than once. Furthermore, not all files support sendfile(), so we
may need to fall back to read/write.

On the read/write path, use write_in_full which deals with short writes.

Signed-off-by: Hristo Venev <hristo@venev.name>
---
 cache.c | 46 ++++++++++++++++++++++++----------------------
 1 file changed, 24 insertions(+), 22 deletions(-)

diff --git a/cache.c b/cache.c
index 55199e8..85cfbd9 100644
--- a/cache.c
+++ b/cache.c
@@ -85,40 +85,42 @@ static int close_slot(struct cache_slot *slot)
 /* Print the content of the active cache slot (but skip the key). */
 static int print_slot(struct cache_slot *slot)
 {
-#ifdef HAVE_LINUX_SENDFILE
-	off_t start_off;
-	int ret;
+	off_t off;
+	ssize_t i;
+
+	off = slot->keylen + 1;
 
-	start_off = slot->keylen + 1;
+#ifdef HAVE_LINUX_SENDFILE
+	off_t size;
+	size = slot->cache_st.st_size;
 
 	do {
-		ret = sendfile(STDOUT_FILENO, slot->cache_fd, &start_off,
-				slot->cache_st.st_size - start_off);
-		if (ret < 0) {
+		i = sendfile(STDOUT_FILENO, slot->cache_fd, &off, size - off);
+		if (i < 0) {
 			if (errno == EAGAIN || errno == EINTR)
 				continue;
+			/* Fall back to read/write on EINVAL */
+			if (errno == EINVAL)
+				break;
 			return errno;
 		}
-		return 0;
+		if (off == size)
+			return 0;
 	} while (1);
-#else
-	ssize_t i, j;
+#endif
 
-	i = lseek(slot->cache_fd, slot->keylen + 1, SEEK_SET);
-	if (i != slot->keylen + 1)
+	if (lseek(slot->cache_fd, off, SEEK_SET) != off)
 		return errno;
 
 	do {
-		i = j = xread(slot->cache_fd, slot->buf, sizeof(slot->buf));
-		if (i > 0)
-			j = xwrite(STDOUT_FILENO, slot->buf, i);
-	} while (i > 0 && j == i);
-
-	if (i < 0 || j != i)
-		return errno;
-	else
-		return 0;
-#endif
+		i = xread(slot->cache_fd, slot->buf, sizeof(slot->buf));
+		if (i < 0)
+			return errno;
+		if (i == 0)
+			return 0;
+		if (write_in_full(STDOUT_FILENO, slot->buf, i) < 0)
+			return errno;
+	} while (1);
 }
 
 /* Check if the slot has expired */
-- 
2.31.1


^ permalink raw reply	[flat|nested] 3+ messages in thread

* Re: [PATCH] cache: Tolerate short writes in print_slot
  2021-09-10 14:18 [PATCH] cache: Tolerate short writes in print_slot Hristo Venev
@ 2021-10-07  9:35 ` John Keeping
  2021-10-07 15:44   ` Hristo Venev
  0 siblings, 1 reply; 3+ messages in thread
From: John Keeping @ 2021-10-07  9:35 UTC (permalink / raw)
  To: Hristo Venev; +Cc: cgit

On Fri, Sep 10, 2021 at 05:18:41PM +0300, Hristo Venev wrote:
> sendfile() can return after a short read/write, so we may need to call
> it more than once. Furthermore, not all files support sendfile(), so we
> may need to fall back to read/write.

Have you seen these errors in practice, or is this just theoretical?

In recent (since v2.6.33) versions of Linux, all files should support
sendfile(), especially since we expect out_fd to be a socket or pipe.

> On the read/write path, use write_in_full which deals with short writes.
> 
> Signed-off-by: Hristo Venev <hristo@venev.name>
> ---
>  cache.c | 46 ++++++++++++++++++++++++----------------------
>  1 file changed, 24 insertions(+), 22 deletions(-)
> 
> diff --git a/cache.c b/cache.c
> index 55199e8..85cfbd9 100644
> --- a/cache.c
> +++ b/cache.c
> @@ -85,40 +85,42 @@ static int close_slot(struct cache_slot *slot)
>  /* Print the content of the active cache slot (but skip the key). */
>  static int print_slot(struct cache_slot *slot)
>  {
> -#ifdef HAVE_LINUX_SENDFILE
> -	off_t start_off;
> -	int ret;
> +	off_t off;
> +	ssize_t i;
> +
> +	off = slot->keylen + 1;
>  
> -	start_off = slot->keylen + 1;
> +#ifdef HAVE_LINUX_SENDFILE
> +	off_t size;

decl-after-stmt if HAVE_LINUX_SENDFILE is set.

> +	size = slot->cache_st.st_size;
>  
>  	do {
> -		ret = sendfile(STDOUT_FILENO, slot->cache_fd, &start_off,
> -				slot->cache_st.st_size - start_off);
> -		if (ret < 0) {
> +		i = sendfile(STDOUT_FILENO, slot->cache_fd, &off, size - off);

Why is ret renamed?  i is normally a loop index variable, using it for
the return value here is strange, please stick with "ret".

> +		if (i < 0) {
>  			if (errno == EAGAIN || errno == EINTR)
>  				continue;
> +			/* Fall back to read/write on EINVAL */
> +			if (errno == EINVAL)
> +				break;
>  			return errno;
>  		}
> -		return 0;
> +		if (off == size)
> +			return 0;
>  	} while (1);
> -#else
> -	ssize_t i, j;
> +#endif
>  
> -	i = lseek(slot->cache_fd, slot->keylen + 1, SEEK_SET);
> -	if (i != slot->keylen + 1)
> +	if (lseek(slot->cache_fd, off, SEEK_SET) != off)
>  		return errno;
>  
>  	do {
> -		i = j = xread(slot->cache_fd, slot->buf, sizeof(slot->buf));
> -		if (i > 0)
> -			j = xwrite(STDOUT_FILENO, slot->buf, i);
> -	} while (i > 0 && j == i);
> -
> -	if (i < 0 || j != i)
> -		return errno;
> -	else
> -		return 0;
> -#endif
> +		i = xread(slot->cache_fd, slot->buf, sizeof(slot->buf));
> +		if (i < 0)
> +			return errno;
> +		if (i == 0)
> +			return 0;
> +		if (write_in_full(STDOUT_FILENO, slot->buf, i) < 0)
> +			return errno;
> +	} while (1);
>  }
>  
>  /* Check if the slot has expired */
> -- 
> 2.31.1
> 

^ permalink raw reply	[flat|nested] 3+ messages in thread

* Re: [PATCH] cache: Tolerate short writes in print_slot
  2021-10-07  9:35 ` John Keeping
@ 2021-10-07 15:44   ` Hristo Venev
  0 siblings, 0 replies; 3+ messages in thread
From: Hristo Venev @ 2021-10-07 15:44 UTC (permalink / raw)
  To: John Keeping; +Cc: cgit

[-- Attachment #1: Type: text/plain, Size: 4359 bytes --]

On Thu, 2021-10-07 at 10:35 +0100, John Keeping wrote:
> Have you seen these errors in practice, or is this just theoretical?
>
> In recent (since v2.6.33) versions of Linux, all files should support
> sendfile(), especially since we expect out_fd to be a socket or pipe.

Even though I haven't seen any errors from sendfile, I'm not sure if it
always works with fuse filesystems and with third-party filesystems
like zfs. The sendfile(2) man page says this:

"Applications may wish to fall back to read(2)/write(2) in the case
where sendfile() fails with EINVAL or ENOSYS."

> > On the read/write path, use write_in_full which deals with short
> > writes.
> > 
> > Signed-off-by: Hristo Venev <hristo@venev.name>
> > ---
> >  cache.c | 46 ++++++++++++++++++++++++----------------------
> >  1 file changed, 24 insertions(+), 22 deletions(-)
> > 
> > diff --git a/cache.c b/cache.c
> > index 55199e8..85cfbd9 100644
> > --- a/cache.c
> > +++ b/cache.c
> > @@ -85,40 +85,42 @@ static int close_slot(struct cache_slot *slot)
> >  /* Print the content of the active cache slot (but skip the key). */
> >  static int print_slot(struct cache_slot *slot)
> >  {
> > -#ifdef HAVE_LINUX_SENDFILE
> > -       off_t start_off;
> > -       int ret;
> > +       off_t off;
> > +       ssize_t i;
> > +
> > +       off = slot->keylen + 1;
> >  
> > -       start_off = slot->keylen + 1;
> > +#ifdef HAVE_LINUX_SENDFILE
> > +       off_t size;
> 
> decl-after-stmt if HAVE_LINUX_SENDFILE is set.

I didn't know that compilers that don't support that exist outside
museums. I will fix it in v2.

> > +       size = slot->cache_st.st_size;
> >  
> >         do {
> > -               ret = sendfile(STDOUT_FILENO, slot->cache_fd,
> > &start_off,
> > -                               slot->cache_st.st_size - start_off);
> > -               if (ret < 0) {
> > +               i = sendfile(STDOUT_FILENO, slot->cache_fd, &off,
> > size - off);
> 
> Why is ret renamed?  i is normally a loop index variable, using it for
> the return value here is strange, please stick with "ret".

I will fix this in v2.

> > +               if (i < 0) {
> >                         if (errno == EAGAIN || errno == EINTR)
> >                                 continue;
> > +                       /* Fall back to read/write on EINVAL */
> > +                       if (errno == EINVAL)
> > +                               break;
> >                         return errno;
> >                 }
> > -               return 0;
> > +               if (off == size)
> > +                       return 0;
> >         } while (1);
> > -#else
> > -       ssize_t i, j;
> > +#endif
> >  
> > -       i = lseek(slot->cache_fd, slot->keylen + 1, SEEK_SET);
> > -       if (i != slot->keylen + 1)
> > +       if (lseek(slot->cache_fd, off, SEEK_SET) != off)
> >                 return errno;
> >  
> >         do {
> > -               i = j = xread(slot->cache_fd, slot->buf, sizeof(slot-
> > >buf));
> > -               if (i > 0)
> > -                       j = xwrite(STDOUT_FILENO, slot->buf, i);
> > -       } while (i > 0 && j == i);
> > -
> > -       if (i < 0 || j != i)
> > -               return errno;
> > -       else
> > -               return 0;
> > -#endif
> > +               i = xread(slot->cache_fd, slot->buf, sizeof(slot-
> > >buf));
> > +               if (i < 0)
> > +                       return errno;
> > +               if (i == 0)
> > +                       return 0;
> > +               if (write_in_full(STDOUT_FILENO, slot->buf, i) < 0)
> > +                       return errno;
> > +       } while (1);
> >  }
> >  
> >  /* Check if the slot has expired */
> > -- 
> > 2.31.1
> > 


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^ permalink raw reply	[flat|nested] 3+ messages in thread

end of thread, other threads:[~2021-10-07 15:44 UTC | newest]

Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
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2021-09-10 14:18 [PATCH] cache: Tolerate short writes in print_slot Hristo Venev
2021-10-07  9:35 ` John Keeping
2021-10-07 15:44   ` Hristo Venev

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