Redefine max_files_per_process to control additionally opened files

Until now max_files_per_process=N limited each backend to open N files in
total (minus a safety factor), even if there were already more files opened in
postmaster and inherited by backends.  Change max_files_per_process to control
how many additional files each process is allowed to open.

The main motivation for this is the patch to add io_method=io_uring, which
needs to open one file for each backend.  Without this patch, even if
RLIMIT_NOFILE is high enough, postmaster will fail in set_max_safe_fds() if
started with a high max_connections.  The cause of the failure is that, until
now, set_max_safe_fds() subtracted the already open files from
max_files_per_process.

Reviewed-by: Noah Misch <[email protected]>
Discussion: https://postgr.es/m/w6uiicyou7hzq47mbyejubtcyb2rngkkf45fk4q7inue5kfbeo@bbfad3qyubvs
Discussion: https://postgr.es/m/CAGECzQQh6VSy3KG4pN1d=h9J=D1rStFCMR+t7yh_Kwj-g87aLQ@mail.gmail.com

1 files changed, 8 insertions, 6 deletions

diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index 62f1185859f..0c3a2a756e7 100644
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -1047,16 +1047,17 @@ set_max_safe_fds(void)
 
 	/*----------
 	 * We want to set max_safe_fds to
-	 *			MIN(usable_fds, max_files_per_process - already_open)
+	 *			MIN(usable_fds, max_files_per_process)
 	 * less the slop factor for files that are opened without consulting
-	 * fd.c.  This ensures that we won't exceed either max_files_per_process
-	 * or the experimentally-determined EMFILE limit.
+	 * fd.c.  This ensures that we won't allow to open more than
+	 * max_files_per_process, or the experimentally-determined EMFILE limit,
+	 * additional files.
 	 *----------
 	 */
 	count_usable_fds(max_files_per_process,
 					 &usable_fds, &already_open);
 
-	max_safe_fds = Min(usable_fds, max_files_per_process - already_open);
+	max_safe_fds = Min(usable_fds, max_files_per_process);
 
 	/*
 	 * Take off the FDs reserved for system() etc.
@@ -1070,9 +1071,10 @@ set_max_safe_fds(void)
 		ereport(FATAL,
 				(errcode(ERRCODE_INSUFFICIENT_RESOURCES),
 				 errmsg("insufficient file descriptors available to start server process"),
-				 errdetail("System allows %d, server needs at least %d.",
+				 errdetail("System allows %d, server needs at least %d, %d files are already open.",
 						   max_safe_fds + NUM_RESERVED_FDS,
-						   FD_MINFREE + NUM_RESERVED_FDS)));
+						   FD_MINFREE + NUM_RESERVED_FDS,
+						   already_open)));
 
 	elog(DEBUG2, "max_safe_fds = %d, usable_fds = %d, already_open = %d",
 		 max_safe_fds, usable_fds, already_open);